= Rollover issues in time sources
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== Table of Contents

* link:#intro[Introduction]
* link:#terminology[Terminology]
* link:#cycles[Major cycles of interest]
* link:#gps_pivots[GPS pivot dates]
* link:#ntp_pivots[NTP pivot dates]

[[intro]]
== Introduction

All time-handling software and hardware suffers from a fundamental
problem: finite-sized data representations for times means that,
sooner or later, your time counter is going to roll over to zero. If
you are lucky, the space you have to represent time is large enough
that you can expect not to observe a rollover during the expected
lifetime of the universe (or whatever portion of it concerns you).
On real-world hardware you will seldom be that lucky.

This page discusses calendar cycles related to NTP, the ways they
interact, the consequences of rollover and common workarounds for
rollover problems.

[[terminology]]
== Terminology

Every calendar cycle has at least three basic properties: a start date
(usually referenced to a solar astronomical date in the Gregorian
calendar), a unit (such as seconds) and a rollover time or cycle
length in that unit.  It also matters what the calendar does about
leap seconds.

As an important example, the basic time representation used on
computers running Unix has a start date of 1970-01-01T00:00:00
(midnight of January 1st 1970), a unit of seconds and a rollover time
of either 2^31^ seconds or 2^63^ seconds, depending on the word size
of the hardware. Negative times represent seconds before the start
date.

The start date is often called the "epoch" and a rollover period may
be called an "era". Beware that eras are often numbered zero-origin;
it is likely that Unix software will refer to dates immediately before
the 2038 rollover as "era 0", not "era 1".

Rollover problems arise when a counter wraps to zero, so that a system
behaves or reports as though time has warped into the far past. The
consequences can range from trivial to catastrophic; time-dependent
software enters previously unexplored regions of its behavior space
and may crash or deliver nonsensical results.  Because these edge
cases are difficult to test, they are defect attractors even when
system designers have been careful and conscientious.

[[cycles]]
== Major cycles of interest

For purposes related to NTP there are four major cyclic calendars of
interest: two-digit years, Unix time, NTP time, GPS time (because GPSes
are so often used as primary time sources).

=== Two-digit year reports

A well-known historical rollover problem was "Y2K", which derived from
the use of two-digit decimal year counters in mainframe software
(written when storage was so limited that longer counters would have
been dramatically more expensive). Expensive preparation and
mitigation work prevented any major disasters in the year 2000, but
this was in part because there were many fewer vulnerable systems than
there later became.

Alas, Y2K-like problems are not dead.  Some will persist as long
as bad old time-source hardware that only reports two-digit years
is in service.

Many GPSes have this problem due to NMEA0183's inadequacies as a
reporting format (and analogous problems in various proprietary GPS
protocols). Unless the device ships a particular $GPZDA sentence
(which many do not) the year parts of time stamps are the low two
digits only.

Dedicated non-GPS reference-clock hardware designed in the 20th
century frequently also report only two-digit year dates - for
example, the Arbiter and Spectracom devices supported by this
distribution.  This problem may persist in post-2000 devices that
are designed for compatibility to use the same reporting protocols,
such as Spectracom's "Type 2".

There is an internal workaround to deal with two-digit dates in
{ntpdman}, but it relies on the system clock being at least roughly
correct - it won't work (for example) at boot time if the clock
has been trashed or zeroed due to a failed battery backup and
you have no remote check peers yet.

Warnings have been attached to the documentation of NTP clock drivers
that may have a vulnerability here.  Notably, the NMEA driver
does *not* have this problem; it
uses a clever calendrical trick to deduce a 4-digit year
that should work until 2399.

=== Unix time

The basics of Unix time have already been described.

32-bit Unix time will roll over at 2038-01-19T03:14:08, beginning its
second era.  It will not roll over to zero but rather to the minimum
*negative* Unix time, which will report as 1901-12-13T20:45:52.

Unix time is leap second-corrected. When a leap second is inserted,
the Unix counter stutters - increases during the leap second, then
drops back a second so that the same counter value represents two
distinct times.  If a leap second were to be deleted, the counter
would skip that second.

The possibility of stutter or skip also means that in order to compute
elapsed time in seconds between two Unix times, you need to know
the leap second offset at each time.  It is not guaranteed that
these offsets are the same if the interval crosses midnight at the
end of any calendar month. To date in 2018, IERS (the International
Earth Rotation service) has performed all corrections just after
midnight of June 31 and December 31 respectively.

The purpose of leap second correction is to keep Unix time
synchronized to the solar calendar.  With it, Unix time after
1972-01-01T00:00:00 (the start date of UTC, Universal Coordinated
Time) coincides with UTC.  Unix time before 1972 was an approximation
of GMT (Greenwich Mean Time) without leap seconds.

On POSIX-compliant Unix systems (which are now effectively all) Unix
time can be retrieved with nanosecond precision; some older versions
supported only millisecond precision.  The subseconds counter is not
involved in any of the rollover or leap second issues with the
seconds counter.

The Unix 32-bit rollover may fizzle the way Y2K did, and will if a
large enough percentage of 32-bit computers are replaced with 64-bit
systems (the time counters on those won't roll over for approximately
292 billion years, which is twenty times the lifetime of the universe
so far).  There is, however, reason for worry; various Unix
filesystems, binary file formats, and databases even on 64-bit systems
use 32-bit fields.  A more serious worry is embedded 32-bit hardware
quietly ticking away in life-critical systems everywhere from medical
devices to avionics.

=== NTP time

NTP time has a 32-bit cycle with an epoch of 1900-01-01T00:00:00.  NTP
time is unsigned; the cycle is thus twice that from a signed 32-bit
counter and the era 1 rollover will be on 2036-01-08.  A detailed
discussion can be found at
https://www.eecis.udel.edu/~mills/y2k.html[The NTP Era and Era
Numbering]. NTP time has a subsecond part in units of 2^-32^
seconds, which does not participate in the seconds counter's rollover
problems.

NTP time is leap second corrected, and the reference implementation
may exhibit stutter or skip.

There is a technique called
https://developers.google.com/time/smear["leap smearing"] used
by some servers that avoids these by changing the length of the issued
second slightly before and after the leap. The AWS equivalent is at
https://aws.amazon.com/blogs/aws/look-before-you-leap-the-coming-leap-second-and-aws/["Look Before You Leap"] .

Leap Smear is supported by NTPSec, please see
link:leapsmear.html[Leap Second Smearing with NTP] .

=== GPS time

Raw GPS time is expressed as weeks since the GPS epoch
1980-01-06T00:00:00, and seconds within the week.  Due to cost
constraints at the time the system was designed, the week counters are
only 10 bits long; thus, GPS time has a period of 1024 weeks (a bit
over 19 years).  At time of writing, two GPS rollovers
have occurred; the first at 1999-08-22T00:00:00 and the second at
2019-04-07T00:00:00.  Note that these times are "GPS Time", not
UTC, the corresponding UTC was 2019-04-06T23:59:42Z , ie a
difference of the 18 leapseconds that had elapsed since the
GPS epoch.

It is planned that future GPS satellites will increase the week
counter length to 13 bits, for a period of 8192 weeks or more
than 157 years.

(GPS was originally designed for military geolocation, not time
service. It does not seem to have occurred to the originators that
long-service GPS clocks would ever be built.)

Raw GPS time does not include subseconds, but does ship a
top-of-second notification accurate to 50ns.  Receivers are expected
to compose this with message data to report time at subsecond
precision.  Many, however, do not, and those that do are often
inaccurate; consumer-grade hardware often has jitter of over a tenth
of a second (100ms). On the other hand, carefully designed GPSes
easily deliver 1ms accuracy, and some do 1000 times better than that. For
a more detailed discussion of accuracy budgets see the
https://gpsd.io/time-service-intro.html[Introduction to Time
Service].

Raw GPS time is not leap second corrected, but the satellite messages
include a leap second offset field in a special update approximately
every 20 minutes.  If the receiver firmware is not carefully designed
there will be a window between device boot time and the next
leap second update, and around leap second corrections, during which
the reported second will be incorrect.

The qualifications about careful design are important because GPS
firmware - especially in inexpensive consumer-grade devices - is
often low-quality and poorly tested due to vendors trying to squeeze
every possible dime out of their costs.  Even major vendors like
SiRF have a history of embarrassing firmware glitches, and the
semi-anonymous outfits in Shenzhen and Taiwan are worse.

Cheaper, consumer-grade, GPS devices also suffer from early EOL, and
manufacturers may never release firmware updates.

The worst impact of careless, low-budget design when using a GPS as
an NTP clock is not around either jitter or leap second corrections,
but rather the behavior near GPS and century rollovers.

[[gps_pivots]]
== GPS pivot dates and rollover compensation

To yield a UTC date, the weeks+second information from the GPS satellites
has to be used as an offset to a base date.

The most naive way to do this would be to simply use the zero date of the
current GPS era (1024-week cycle) as the base.  Some very early GPS
equipment seems to have worked this way, but it has the drawback that
the device time-warps at the next GPS era rollover - which, if your
device first shipped only a few years before that rollover is a
serious drawback.

There is a relatively simple way to extend the useful life of a GPS
used as a clock. That is to choose a pivot date in GPS weeks.  When
your device reports a week number *before* the pivot, you assume that
a GPS rollover has occurred and add 1024 weeks to the base date.

This technique will postpone time-warping to a full 1024 weeks after
the pivot date - which the vendor typically makes the release date
of the device firmware.  But it won't do any better than that; without the
ability to reconfigure the base date, the device will ship incorrect
reports forever afterwards.  A few refclocks, like the HP-GPS line,
have that ability; most do not.

This is where careless, low-budget design begins to matter. GPS vendors
do not document even the fact that they use base or pivot dates, let
alone what the base and pivot dates for their devices are. As of 2018,
NTP's developers do not know of any devices for which you can even
query these parameters, let alone set them.  The best you can generally
do (and on only some devices) is get the firmware release date; from this,
you can assume that you will get non-timewarped reports for 1024 weeks
after that and probably be right.

This is not entirely the GPS vendors' fault.  The truth is, GPS
reporting protocols are an awful mess.  The closest thing to a
standard is NMEA0183, originally designed for a different purpose; it
is poorly specified and has no standard device-control functions at
all, let alone any for querying base and pivot dates.
https://gpsd.io/[GPSD], the ubiquitous open-source GPS
manager daemon that shares some developers with NTPsec, works around a
lot of the general messiness, but can't solve this problem
because the device capabilities to address it are simply absent.

Finally, when you buy a GPS device, you do not know - and often will not be
able to discover - how far in the past its hidden pivot date is.  This
means that time service operators using a GPS as a local Stratum 0
need to be aware of the possibility that their device could roll over
at any time (but especially near the zero days of a GPS era) and be
ready to compensate by dropping in a device with a newer pivot date.

Actionable advice that follows from this: If you don't know the
manufacturing date of a GPS-based device to within a couple of years,
don't trust that it won't be rolled over *the first time
you start it*.  Shopping for cheap refclocks at surplus houses or
on the Web is particularly hazardous this way.

NTPsec includes a workaround option for these old devices. Each "g"
suffix on the 'time1' offset option for a device adds the number of seconds
in a 1024-week era; this corresponds to the 10-bit week counters used
in today's GNSS. "G"  adds the number of seconds in the 8192-week era
of the future.  Unfortunately, this will only be useful *after* you
detect a rollover.

A fairly bulletproof mitigation strategy, if you are running
autonomously, is to have multiple GPS clocks per server and replace
each individual one at least once every nine years (half a GPS era).

[[ntp_pivots]]
== NTP pivot dates

For purposes of this discussion, a 'resolved' timestamp is one that
has been mapped from a 32-bit seconds offset in some era to a full
64-bit timestamp implicitly based in some known era.

When {ntpdman} receives an unresolved timestamp from an upstream server,
that timestamp could be based in any era prior to the current
wall-clock date - but by definition we may not know the wall-clock
date with confidence yet (not having achieved sync).

To resolve this ambiguity, NTP also uses an internal pivot date.  It
chooses the era that would minimize the absolute value of the time
difference between the resolved timestamp and its internal pivot date.

This resolution technique is part of the NTP specification. It will
normally resolve to the current era, but may in cases where the pivot
is within a half-cycle of a rollover time resolve to either the
previous or the next era.

An {ntpdman} instance's pivot date will be the date it was compiled
and built.

It is fairly easy to see that this guarantees correct resolution if the
pivot dates of the communicating ntpds are within a half-cycle
(88 years) of each other, even if there is a rollover between the pivot
dates.

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