Go to the first, previous, next, last section, table of contents.

Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what the current time is and conversion between different time representations.

The time functions fall into three main categories:

• Functions for measuring elapsed CPU time are discussed in section Processor Time.
• Functions for measuring absolute clock or calendar time are discussed in section Calendar Time.
• Functions for setting alarms and timers are discussed in section Setting an Alarm.

Processor Time

If you're trying to optimize your program or measure its efficiency, it's very useful to be able to know how much processor time or CPU time it has used at any given point. Processor time is different from actual wall clock time because it doesn't include any time spent waiting for I/O or when some other process is running. Processor time is represented by the data type clock_t, and is given as a number of clock ticks relative to an arbitrary base time marking the beginning of a single program invocation.

Basic CPU Time Inquiry

To get the elapsed CPU time used by a process, you can use the clock function. This facility is declared in the header file `time.h'.

In typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values, and then divide by CLOCKS_PER_SEC (the number of clock ticks per second), like this:

#include <time.h>

clock_t start, end;
double elapsed;

start = clock();
... /* Do the work. */
end = clock();
elapsed = ((double) (end - start)) / CLOCKS_PER_SEC;

Different computers and operating systems vary wildly in how they keep track of processor time. It's common for the internal processor clock to have a resolution somewhere between hundredths and millionths of a second.

In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the type of the macro CLOCKS_PER_SEC can be either integer or floating-point types. Casting processor time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Macro: int CLOCKS_PER_SEC

The value of this macro is the number of clock ticks per second measured by the clock function.

Macro: int CLK_TCK

This is an obsolete name for CLOCKS_PER_SEC.

Data Type: clock_t

This is the type of the value returned by the clock function. Values of type clock_t are in units of clock ticks.

Function: clock_t clock (void)

This function returns the elapsed processor time. The base time is arbitrary but doesn't change within a single process. If the processor time is not available or cannot be represented, clock returns the value (clock_t)(-1).

Detailed Elapsed CPU Time Inquiry

The times function returns more detailed information about elapsed processor time in a struct tms object. You should include the header file `sys/times.h' to use this facility.

Data Type: struct tms

The tms structure is used to return information about process times. It contains at least the following members:

clock_t tms_utime

This is the CPU time used in executing the instructions of the calling process.

clock_t tms_stime

This is the CPU time used by the system on behalf of the calling process.

clock_t tms_cutime

This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid; see section Process Completion. In other words, it represents the total CPU time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by wait or waitpid.

clock_t tms_cstime

This is similar to tms_cutime, but represents the total CPU time used by the system on behalf of all the terminated child processes of the calling process.

All of the times are given in clock ticks. These are absolute values; in a newly created process, they are all zero. See section Creating a Process.

Function: clock_t times (struct tms *buffer)

The times function stores the processor time information for the calling process in buffer.

The return value is the same as the value of clock(): the elapsed real time relative to an arbitrary base. The base is a constant within a particular process, and typically represents the time since system start-up. A value of (clock_t)(-1) is returned to indicate failure.

Portability Note: The clock function described in section Basic CPU Time Inquiry, is specified by the ISO C standard. The times function is a feature of POSIX.1. In the GNU system, the value returned by the clock function is equivalent to the sum of the tms_utime and tms_stime fields returned by times.

Calendar Time

This section describes facilities for keeping track of dates and times according to the Gregorian calendar.

There are three representations for date and time information:

• Calendar time (the time_t data type) is a compact representation, typically giving the number of seconds elapsed since some implementation-specific base time.
• There is also a high-resolution time representation (the struct timeval data type) that includes fractions of a second. Use this time representation instead of ordinary calendar time when you need greater precision.
• Local time or broken-down time (the struct tm data type) represents the date and time as a set of components specifying the year, month, and so on, for a specific time zone. This time representation is usually used in conjunction with formatting date and time values.

Simple Calendar Time

This section describes the time_t data type for representing calendar time, and the functions which operate on calendar time objects. These facilities are declared in the header file `time.h'.

Data Type: time_t

This is the data type used to represent calendar time. When interpreted as an absolute time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This date is sometimes referred to as the epoch.) POSIX requires that this count ignore leap seconds, but on some hosts this count includes leap seconds if you set TZ to certain values (see section Specifying the Time Zone with TZ).

In the GNU C library, time_t is equivalent to long int. In other systems, time_t might be either an integer or floating-point type.

Function: double difftime (time_t time1, time_t time0)

The difftime function returns the number of seconds elapsed between time time1 and time time0, as a value of type double. The difference ignores leap seconds unless leap second support is enabled.

In the GNU system, you can simply subtract time_t values. But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

Function: time_t time (time_t *result)

The time function returns the current time as a value of type time_t. If the argument result is not a null pointer, the time value is also stored in *result. If the calendar time is not available, the value (time_t)(-1) is returned.

High-Resolution Calendar

The time_t data type used to represent calendar times has a resolution of only one second. Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in `sys/time.h'.

Data Type: struct timeval

The struct timeval structure represents a calendar time. It has the following members:

long int tv_sec

This represents the number of seconds since the epoch. It is equivalent to a normal time_t value.

long int tv_usec

This is the fractional second value, represented as the number of microseconds. Some times struct timeval values are used for time intervals. Then the tv_sec member is the number of seconds in the interval, and tv_usec is the number of additional microseconds.

Data Type: struct timezone

The struct timezone structure is used to hold minimal information about the local time zone. It has the following members:

int tz_minuteswest

This is the number of minutes west of UTC.

int tz_dsttime

If nonzero, daylight saving time applies during some part of the year.

The struct timezone type is obsolete and should never be used. Instead, use the facilities described in section Functions and Variables for Time Zones.

It is often necessary to subtract two values of type struct timeval. Here is the best way to do this. It works even on some peculiar operating systems where the tv_sec member has an unsigned type.

/* Subtract the `struct timeval' values X and Y,
   storing the result in RESULT.
   Return 1 if the difference is negative, otherwise 0.  */

int
timeval_subtract (result, x, y)
     struct timeval *result, *x, *y;
{
  /* Perform the carry for the later subtraction by updating y. */
  if (x->tv_usec < y->tv_usec) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
    y->tv_usec -= 1000000 * nsec;
    y->tv_sec += nsec;
  }
  if (x->tv_usec - y->tv_usec > 1000000) {
    int nsec = (y->tv_usec - x->tv_usec) / 1000000;
    y->tv_usec += 1000000 * nsec;
    y->tv_sec -= nsec;
  }

  /* Compute the time remaining to wait.
     tv_usec is certainly positive. */
  result->tv_sec = x->tv_sec - y->tv_sec;
  result->tv_usec = x->tv_usec - y->tv_usec;

  /* Return 1 if result is negative. */
  return x->tv_sec < y->tv_sec;
}

Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)

The gettimeofday function returns the current date and time in the struct timeval structure indicated by tp. Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

ENOSYS

The operating system does not support getting time zone information, and tzp is not a null pointer. The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in section Functions and Variables for Time Zones.

Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)

The settimeofday function sets the current date and time according to the arguments. As for gettimeofday, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use settimeofday.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM

This process cannot set the time because it is not privileged.

ENOSYS

The operating system does not support setting time zone information, and tzp is not a null pointer.

Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)

This function speeds up or slows down the system clock in order to make gradual adjustments in the current time. This ensures that the time reported by the system clock is always monotonically increasing, which might not happen if you simply set the current time.

The delta argument specifies a relative adjustment to be made to the current time. If negative, the system clock is slowed down for a while until it has lost this much time. If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it. The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM

You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday, and adjtime functions are derived from BSD.

Broken-down Time

Calendar time is represented as a number of seconds. This is convenient for calculation, but has no resemblance to the way people normally represent dates and times. By contrast, broken-down time is a binary representation separated into year, month, day, and so on. Broken down time values are not useful for calculations, but they are useful for printing human readable time.

A broken-down time value is always relative to a choice of local time zone, and it also indicates which time zone was used.

The symbols in this section are declared in the header file `time.h'.

Data Type: struct tm

This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order:

int tm_sec

This is the number of seconds after the minute, normally in the range 0 through 59. (The actual upper limit is 60, to allow for leap seconds if leap second support is available.)

int tm_min

This is the number of minutes after the hour, in the range 0 through 59.

int tm_hour

This is the number of hours past midnight, in the range 0 through 23.

int tm_mday

This is the day of the month, in the range 1 through 31.

int tm_mon

This is the number of months since January, in the range 0 through 11.

int tm_year

This is the number of years since 1900.

int tm_wday

This is the number of days since Sunday, in the range 0 through 6.

int tm_yday

This is the number of days since January 1, in the range 0 through 365.

int tm_isdst

This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.

long int tm_gmtoff

This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is -5*60*60. The tm_gmtoff field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.

const char *tm_zone

This field is the name for the time zone that was used to compute this broken-down time value. Like tm_gmtoff, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

Function: struct tm * localtime (const time_t *time)

The localtime function converts the calendar time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to ctime, gmtime, or localtime. (But no other library function overwrites the contents of this object.)

The return value is the null pointer if time cannot be represented as a broken-down time; typically this is because the year cannot fit into an int.

Calling localtime has one other effect: it sets the variable tzname with information about the current time zone. See section Functions and Variables for Time Zones.

Using the localtime function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a varient of this function.

Function: struct tm * localtime_r (const time_t *time, struct tm *resultp)

The localtime_r function works just like the localtime function. It takes a pointer to a variable containing the calendar time and converts it to the broken-down time format.

But the result is not placed in a static buffer. Instead it is placed in the object of type struct tm to which the parameter resultp points.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: struct tm * gmtime (const time_t *time)

This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC)---that is, as Greenwich Mean Time (GMT)---rather than relative to the local time zone.

Recall that calendar times are always expressed in coordinated universal time.

As for the localtime function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for gmtime.

Function: struct tm * gmtime_r (const time_t *time, struct tm *resultp)

This function is similar to localtime_r, except that it converts just like gmtime the given time as Coordinated Universal Time.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

Function: time_t mktime (struct tm *brokentime)

The mktime function is used to convert a broken-down time structure to a calendar time representation. It also "normalizes" the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure. It uses the values of the other components to compute the calendar time; it's permissible for these components to have unnormalized values outside of their normal ranges. The last thing that mktime does is adjust the components of the brokentime structure (including the tm_wday and tm_yday).

If the specified broken-down time cannot be represented as a calendar time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.

Calling mktime also sets the variable tzname with information about the current time zone. See section Functions and Variables for Time Zones.

Formatting Date and Time

The functions described in this section format time values as strings. These functions are declared in the header file `time.h'.

Function: char * asctime (const struct tm *brokentime)

The asctime function converts the broken-down time value that brokentime points to into a string in a standard format:

"Tue May 21 13:46:22 1991\n"

The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.

The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.

The return value points to a statically allocated string, which might be overwritten by subsequent calls to asctime or ctime. (But no other library function overwrites the contents of this string.)

Function: char * asctime_r (const struct tm *brokentime, char *buffer)

This function is similar to asctime but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter buffer. This buffer should have at least room for 16 bytes.

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

Function: char * ctime (const time_t *time)

The ctime function is similar to asctime, except that the time value is specified as a time_t calendar time value rather than in broken-down local time format. It is equivalent to

asctime (localtime (time))

ctime sets the variable tzname, because localtime does so. See section Functions and Variables for Time Zones.

Function: char * ctime_r (const time_t *time, char *buffer)

This function is similar to ctime, only that it places the result in the string pointed to by buffer. It is equivalent to (written using gcc extensions, see section `Statement Exprs' in Porting and Using gcc):

({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)

This function is similar to the sprintf function (see section Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see section Locales and Internationalization).

Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:

_

The number is padded with spaces.

-

The number is not padded at all.

0

The number is padded with zeros even if the format specifies padding with spaces.

^

The output uses uppercase characters, but only if this is possible (see section Case Conversion).

The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.

Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.

An optional modifier can follow the optional flag and width specification. The modifiers, which are POSIX.2 extensions, are:

E

Use the locale's alternate representation for date and time. This modifier applies to the %c, %C, %x, %X, %y and %Y format specifiers. In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns.

O

Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers.

If the format supports the modifier but no alternate representation is available, it is ignored.

The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows:

%a

The abbreviated weekday name according to the current locale.

%A

The full weekday name according to the current locale.

%b

The abbreviated month name according to the current locale.

%B

The full month name according to the current locale.

%c

The preferred date and time representation for the current locale.

%C

The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100. This format is a POSIX.2 extension.

%d

The day of the month as a decimal number (range 01 through 31).

%D

The date using the format %m/%d/%y. This format is a POSIX.2 extension.

%e

The day of the month like with %d, but padded with blank (range 1 through 31). This format is a POSIX.2 extension.

%f

The day of the week as a decimal number (range 1 through 7), Monday being 1. This format is a ISO C 9X extension.

%F

The date using the format %Y-%m-%d. This is the form specified in the ISO 8601 standard and is the preferred form for all uses. This format is a ISO C 9X extension.

%g

The year corresponding to the ISO week number, but without the century (range 00 through 99). This has the same format and value as %y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead. This format is a GNU extension.

%G

The year corresponding to the ISO week number. This has the same format and value as %Y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead. This format is a GNU extension.

%h

The abbreviated month name according to the current locale. The action is the same as for %b. This format is a POSIX.2 extension.

%H

The hour as a decimal number, using a 24-hour clock (range 00 through 23).

%I

The hour as a decimal number, using a 12-hour clock (range 01 through 12).

%j

The day of the year as a decimal number (range 001 through 366).

%k

The hour as a decimal number, using a 24-hour clock like %H, but padded with blank (range 0 through 23). This format is a GNU extension.

%l

The hour as a decimal number, using a 12-hour clock like %I, but padded with blank (range 1 through 12). This format is a GNU extension.

%m

The month as a decimal number (range 01 through 12).

%M

The minute as a decimal number (range 00 through 59).

%n

A single `\n' (newline) character. This format is a POSIX.2 extension.

%p

Either `AM' or `PM', according to the given time value; or the corresponding strings for the current locale. Noon is treated as `PM' and midnight as `AM'.

%P

Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as `pm' and midnight as `am'. This format is a GNU extension.

%r

The complete time using the AM/PM format of the current locale. This format is a POSIX.2 extension.

%R

The hour and minute in decimal numbers using the format %H:%M. This format is a GNU extension.

%s

The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. This format is a GNU extension.

%S

The second as a decimal number (range 00 through 60).

%t

A single `\t' (tabulator) character. This format is a POSIX.2 extension.

%T

The time using decimal numbers using the format %H:%M:%S. This format is a POSIX.2 extension.

%u

The day of the week as a decimal number (range 1 through 7), Monday being 1. This format is a POSIX.2 extension.

%U

The week number of the current year as a decimal number (range 00 through 53), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week 00.

%V

The ISO 8601:1988 week number as a decimal number (range 01 through 53). ISO weeks start with Monday and end with Sunday. Week 01 of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week 01 of a year can contain days from the previous year. The week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year. This format is a POSIX.2 extension.

%w

The day of the week as a decimal number (range 0 through 6), Sunday being 0.

%W

The week number of the current year as a decimal number (range 00 through 53), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week 00.

%x

The preferred date representation for the current locale, but without the time.

%X

The preferred time representation for the current locale, but with no date.

%y

The year without a century as a decimal number (range 00 through 99). This is equivalent to the year modulo 100.

%Y

The year as a decimal number, using the Gregorian calendar. Years before the year 1 are numbered 0, -1, and so on.

%z

RFC 822/ISO 8601:1988 style numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable. This format is a GNU extension. A full RFC 822 timestamp is generated by the format `"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent `"%a, %d %b %Y %T %z"'.

%Z

The time zone abbreviation (empty if the time zone can't be determined).

%%

A literal `%' character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character. If the formatted time requires more than size characters, strftime returns zero and the content of the array s is indetermined. Otherwise the return value indicates the number of characters placed in the array s, not including the terminating null character.

Warning: This convention for the return value which is prescribed in ISO C can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales "%p" will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used:

buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
  {
    /* Something went wrong in the strftime call.  */
    ...
  }

If s is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.

According to POSIX.1 every call to strftime implies a call to tzset. So the contents of the environment variable TZ is examined before any output is produced.

For an example of strftime, see section Time Functions Example.

Specifying the Time Zone with TZ

In POSIX systems, a user can specify the time zone by means of the TZ environment variable. For information about how to set environment variables, see section Environment Variables. The functions for accessing the time zone are declared in `time.h'.

You should not normally need to set TZ. If the system is configured properly, the default time zone will be correct. You might set TZ if you are using a computer over the network from a different time zone, and would like times reported to you in the time zone that local for you, rather than what is local for the computer.

In POSIX.1 systems the value of the TZ variable can be of one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database.

The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone:

std offset

The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon or embedded digits, commas, or plus or minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.

The offset specifies the time value one must add to the local time to get a Coordinated Universal Time value. It has syntax like [+|-]hh[:mm[:ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 23, and the minute and seconds between 0 and 59.

For example, here is how we would specify Eastern Standard Time, but without any daylight saving time alternative:

EST+5

The second format is used when there is Daylight Saving Time:

std offset dst [offset],start[/time],end[/time]

The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding daylight saving time zone; if the offset is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when daylight saving time is in effect. The start field is when daylight saving time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:

Jn

This specifies the Julian day, with n between 1 and 365. February 29 is never counted, even in leap years.

n

This specifies the Julian day, with n between 0 and 365. February 29 is counted in leap years.

Mm.w.d

This specifies day d of week w of month m. The day d must be between 0 (Sunday) and 6. The week w must be between 1 and 5; week 1 is the first week in which day d occurs, and week 5 specifies the last d day in the month. The month m should be between 1 and 12.

The time fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is 02:00:00.

For example, here is how one would specify the Eastern time zone in the United States, including the appropriate daylight saving time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am.

EST+5EDT,M4.1.0/2,M10.5.0/2

The schedule of daylight saving time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule--usually the present day schedule--and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below).

The third format looks like this:

:characters

Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification `TZ=:/etc/localtime' (or `TZ=:/usr/local/etc/localtime', depending on how GNU C library was configured; see section Installing the GNU C Library). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

If characters begins with a slash, it is an absolute file name; otherwise the library looks for the file `/share/lib/zoneinfo/characters'. The `zoneinfo' directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, `America/New_York', `Europe/London', `Asia/Hong_Kong'. These data files are installed by the system administrator, who also sets `/etc/localtime' to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.

Functions and Variables for Time Zones

Variable: char * tzname [2]

The array tzname contains two strings, which are the standard names of the pair of time zones (standard and daylight saving) that the user has selected. tzname[0] is the name of the standard time zone (for example, "EST"), and tzname[1] is the name for the time zone when daylight saving time is in use (for example, "EDT"). These correspond to the std and dst strings (respectively) from the TZ environment variable. If daylight saving time is never used, tzname[1] is the empty string.

The tzname array is initialized from the TZ environment variable whenever tzset, ctime, strftime, mktime, or localtime is called. If multiple abbreviations have been used (e.g. "EWT" and "EDT" for U.S. Eastern War Time and Eastern Daylight Time), the array contains the most recent abbreviation.

The tzname array is required for POSIX.1 compatibility, but in GNU programs it is better to use the tm_zone member of the broken-down time structure, since tm_zone reports the correct abbreviation even when it is not the latest one.

Though the strings are declared as char * the user must stay away from modifying these strings. Modifying the strings will almost certainly lead to trouble.

Function: void tzset (void)

The tzset function initializes the tzname variable from the value of the TZ environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix. Like tzname, these variables are set by calling tzset or the other time conversion functions.

Variable: long int timezone

This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is 5*60*60. Unlike the tm_gmtoff member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use tm_gmtoff, since it contains the correct offset even when it is not the latest one.

Variable: int reverse> through 12). This format is a GNU extension.

%m

The month as a decimal number (range 01 through 12).

%M

The minute as a decimal number (range 00 through 59).

%n

A single `\n' (newline) character. This format is a POSIX.2 extension.

%p

Either `AM' or `PM', according to the given time value; or the corresponding strings for the current locale. Noon is treated as `PM' and midnight as `AM'.

%P

Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as `pm' and midnight as `am'. This format is a GNU extension.

%r

The complete time using the AM/PM format of the current locale. This format is a POSIX.2 extension.

%R

The hour and minute in decimal numbers using the format %H:%M. This format is a GNU extension.

%s

The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available. This format is a GNU extension.

%S

The second as a decimal number (range 00 through 60).

%t

A single `\t' (tabulator) character. This format is a POSIX.2 extension.

%T

The time using decimal numbers using the format %H:%M:%S. This format is a POSIX.2 extension.

%u

The day of the week as a decimal number (range 1 through 7), Monday being 1. This format is a POSIX.2 extension.

%U

The week number of the current year as a decimal number (range 00 through 53), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week 00.

%V

The ISO 8601:1988 week number as a decimal number (range 01 through 53). ISO weeks start with Monday and end with Sunday. Week 01 of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week 01 of a year can contain days from the previous year. The week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year. This format is a POSIX.2 extension.

%w

The day of the week as a decimal number (range 0 through 6), Sunday being 0.

%W

The week number of the current year as a decimal number (range 00 through 53), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week 00.

%x

The preferred date representation for the current locale, but without the time.

%X

The preferred time representation for the current locale, but with no date.

%y

The year without a century as a decimal number (range 00 through 99). This is equivalent to the year modulo 100.

%Y

The year as a decimal number, using the Gregorian calendar. Years before the year 1 are numbered 0, -1, and so on.

%z

RFC 822/ISO 8601:1988 style numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable. This format is a GNU extension. A full RFC 822 timestamp is generated by the format `"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent `"%a, %d %b %Y %T %z"'.

%Z

The time zone abbreviation (empty if the time zone can't be determined).

%%

A literal `%' character.

buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
  {
    /* Something went wrong in the strftime call.  */
    ...
  }

std offset

EST+5

std offset dst [offset],start[/time],end[/time]

EST+5EDT,M4.1.0/2,M10.5.0/2

:characters

buf[0] = '\1';
len = strftime (buf, bufsize, format, tp);
if (len == 0 && buf[0] != '\0')
  {
    /* Something went wrong in the strftime call.  */
    ...
  }

std offset

EST+5

std offset dst [offset],start[/time],end[/time]

EST+5EDT,M4.1.0/2,M10.5.0/2

:characters