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Versions: 00                                                            
INTERNET-DRAFT                                                   M. Kuhn
draft-kuhn-leapsecond-00.txt                     University of Cambridge
Expires: July 2006                                          January 2006

    Coordinated Universal Time with Smoothed Leap Seconds (UTC-SLS)

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   have been or will be disclosed, and any of which he or she becomes
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   Coordinated Universal Time (UTC) is the international standard
   timescale used in many Internet protocols.  UTC features occasional
   single-second adjustments, known as "leap seconds".  These happen at
   the end of announced UTC days, in the form of either an extra second
   23:59:60 or a missing second 23:59:59.  Both events need special
   consideration in UTC-synchronized systems that represent time as a
   scalar value.  This specification defines UTC-SLS, a minor variation
   of UTC that lacks leap seconds.  Instead, UTC-SLS performs an
   equivalent "smooth" adjustment, during which the rate of the clock
   temporarily changes by 0.1% for 1000 seconds.  UTC-SLS is a drop-in
   replacement for UTC.  UTC-SLS can be generated from the same
   information as UTC.  It can be used with any specification that
   refers to UTC but lacks provisions for leap seconds.  UTC-SLS
   provides a robust and interoperable way for networked UTC-

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   synchronized clocks to handle leap seconds.  By providing UTC-SLS
   instead of UTC to applications, operating systems can free most
   application and protocol designers from any need to even know about
   UTC leap seconds.

1  Introduction

   Coordinated Universal Time (UTC) is an internationally agreed precise
   definition of time.  Like its predecessor Greenwich Mean Time (GMT),
   UTC approximates the local mean solar time on Earth's prime meridian,
   which passes through Greenwich in London, England.

   UTC is commonly available through radio time signals [ITU-768],
   global navigation satellite systems, and the Internet [RFC1305], with
   an accuracy ranging from a few milliseconds to a few nanoseconds.  It
   is the basis of official time in many countries, where local time is
   legally defined to differ from UTC exactly by an integral number of
   half hours.  UTC is commonly referred to in the specifications of
   computer applications, communication protocols [ISO8601, RFC3339] and
   application-program interfaces [C, POSIX].

   UTC is defined and maintained by an internationally coordinated
   network of precision clocks ("atomic clocks").  The reference
   frequency they generate is far more stable than Earth's daily
   rotation, on which the traditional astronomical definitions of time
   are based.  In the interest of backwards compatibility, UTC was
   defined such that an adjustment is made before UTC and a particular
   definition of astronomical time, known as UT1 [McC91], drift apart
   more than 0.9 seconds [ITU-460].  These adjustments have, in their
   current form, been applied since 1972 and are known as "leap seconds"
   [Nel01, IERS-C].

   The definition of UTC [ITU-460] provides for two types of adjustment:

     - The "insertion of a second" or "positive leap second" is the
       addition of a 86401-st second denoted "23:59:60" at the end of a
       UTC day.

     - The "deletion of a second" or "negative leap second" is the
       skipping of the last, 86400-th second denoted "23:59:59" at the
       end of a UTC day.

   Time is traditionally represented as a vector of integer numbers,
   denoting year, month, day, hour, minute, second, and some fraction of
   a second, for example written as YYYY-MM-DD hh:mm:ss.sss [ISO8601,
   RFC3339].  A normal UTC day is a progression of 86400 seconds, which
   are numbered 00:00:00 to 23:59:59.

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   When the rotation of Earth has fallen behind UTC, the insertion of a
   second into UTC (positive leap second) will be announced in [IERS-C].
   On the announced date, the UTC day will be 86401 seconds long and a
   UTC clock will display at its end the sequence of seconds

    ..., 23:59:57, 23:59:58, 23:59:59, 23:59:60, 00:00:00, 00:00:01, ...

   Likewise, should the rotation of Earth rush ahead of UTC, the
   deletion of a leap second will be announced, and on that date, the
   UTC day will be only 86399 seconds long and will end with the
   sequence of seconds

    ..., 23:59:57, 23:59:58, 00:00:00, 00:00:01, ...

   Time distribution services, such as NTP [RFC1305], provide
   announcements of forthcoming leap seconds.  These permit clocks
   synchronized to such a time service to display leap seconds exactly
   as defined in [ITU-460, IERS-C] and shown above.

   Leap seconds were introduced because they provide the most convenient
   way of adjusting UTC for some applications, in particular those that
   distribute time via pulse-per-second signals.  By inserting or
   deleting an entire second, such signals are not disturbed, and
   neither are any of the precision frequencies that are generated from
   them by frequency multiplication, including carrier frequencies of
   radio transmitters, television signal timings, etc.

   In other applications, however, leap seconds are less convenient.
   Unlike humans, computers deal with time more easily as a single
   scalar value, rather than as a vector of integers.

   A typical and prominent example of a scalar encoding of time is the
   "seconds since the epoch" timescale.  It is defined in [POSIX] as a
   scalar encoding of a YYYY-MM-DD hh:mm:ss.sss value shown on a clock
   that approximates UTC.  If UTC had no leap seconds, this value would
   represent the number of seconds that have elapsed since the start of
   the UTC day 1970-01-01.

   If a clock is closely synchronized with UTC and receives leap-second
   announcements in advance, then [ITU-460] defines only what happens to
   an hh:mm:ss display near that leap second.  However, if such a clock
   is required to output a scalar time, its implementor and users will
   face two problems.  Firstly, typical scalar encodings of time have no
   unambiguous representation for points in time during a positive leap
   second.  Secondly, both positive and negative leap seconds will cause
   a discontinuity in the scalar representation of time that may be
   disruptive for some applications.

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   [ITU-460] provides no guidance for handling scalar representations of
   time across leap seconds.  [POSIX] leaves the exact behavior of its
   "seconds since the epoch" timescale implementation-defined.  Most
   other protocol and API specifications remain equally silent on the

   This specification fills that gap by providing clock implementors
   with guidance on what a UTC-synchronized computer clock should output
   near a leap second.  It specifies the exact behavior of a clock that
   displays a variant of UTC called UTC-SLS (UTC with Smoothed Leap

2  Application of UTC-SLS

   UTC-SLS is intended as a drop-in replacement for UTC in any
   application protocol interface or communication protocol that does
   not explicitely specify any particular behavior near a leap second.

   UTC-SLS avoids the problems that arise when the UTC clock defined in
   [ITU-460] is converted into a scalar representation of time.  It can
   be used by implementors of UTC-synchronized clocks with scalar output
   in order to achieve interoperable behavior with a low risk of
   disruption in the presence of leap seconds.

   UTC-SLS should be used in computer applications and protocols
   independent of whether they represent time as a scalar value or in
   hh:mm:ss notation.  Even though the hh:mm:ss representation is, in
   principle, capable of encoding inserted UTC leap seconds
   unambiguously, using the 23:59:60 notation from [ITU-460], in
   practice this is not feasible, because hh:mm:ss and scalar
   representations have to be converted into each other frequently.

   If UTC-synchronized computer clocks provide their users routinely
   with an approximation of UTC-SLS instead of an approximation of UTC,
   then it can be hoped that far fewer specification authors and
   software developers will have to be aware of leap seconds.  Leap
   seconds can remain a concern of the time-keeping specialists that
   implement the clock drivers in operating systems and the protocols
   used to synchronize these with external time signals.

   UTC-SLS is not intended to be used as a drop-in replacement in
   specifications that are themselves concerned with the accurate
   synchronization of clocks and that have already an exactly specified
   behavior near UTC leap seconds (e.g., NTP [RFC1305], PTP [IEC1588]).

   Some "real time" applications have very demanding clock-stability
   requirements, for which neither UTC nor UTC-SLS are suitable.
   Appendix B discusses application limits of UTC-SLS and alternatives.

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3  Properties of UTC-SLS

   On a UTC-SLS clock

     - the time of day always progresses through 86400 different
       hh:mm:ss values, and always ends with ..., 23:59:58, 23:59:59,
       followed by 00:00:00, 00:00:01, ...;

     - the time 23:59:60 never appears;

     - the time never differs from UTC by more than one second;

     - the time always equals UTC at full or half hours;

     - the rate can deviate by exactly +/- 0.1% (+/- 1000 ppm).

4  Definition of UTC-SLS

   A UTC-SLS clock shows the exact same time as a UTC clock, except
   during the last 1000 seconds of any UTC day that is extended or
   shortened through the insertion or deletion of one leap second at the
   end of that day.

   4.1  Positive leap second

   Whenever a UTC day is extended by an inserted second, during this
   positive leap second, values of the form 23:59:60.xxxx are displayed
   on a UTC clock, but no such timestamps appear on a UTC-SLS clock.
   Instead, during the last 1000 seconds of that UTC day, starting at
   23:43:21, the UTC-SLS clock slows down to 0.999 times its normal
   rate, which means that the UTC-SLS clock progresses during that time
   only through 999 "slow seconds", each of which lasts 1000/999

   The following table shows the exact and simultaneous display of a UTC
   and UTC-SLS clock at various points in time near an inserted leap

          UTC           UTC-SLS                  Remarks

     23:43:20.0000   23:43:20.0000
     23:43:21.0000   23:43:21.0000   UTC-SLS starts to diverge from UTC
     23:43:22.0000   23:43:21.9990   UTC-SLS is now 1 ms behind UTC
     23:43:23.0000   23:43:22.9980   UTC-SLS is now 2 ms behind UTC
     23:43:24.0000   23:43:23.9970   UTC-SLS is now 3 ms behind UTC
     ... 995 seconds later ...
     23:59:59.0000   23:59:58.0020   UTC-SLS is now 998 ms behind UTC
     23:59:60.0000   23:59:59.0010   UTC leap second starts

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     00:00:00.0000   00:00:00.0000   UTC leap second ends, UTC-SLS = UTC
     00:00:01.0000   00:00:01.0000

   Intermediate UTC-SLS values are defined through linear interpolation
   accordingly, for example:

          UTC           UTC-SLS

     23:43:21.1000   23:43:21.0999
     23:43:21.2000   23:43:21.1998
     23:59:60.9000   23:59:59.9001

   4.2  Negative leap second

   Whenever a UTC day is shortened by deleting a leap second, no values
   of the form 23:59:59.xxxx are displayed on a UTC clock, but all these
   timestamps nevertheless appear on a UTC-SLS clock.  Instead, during
   the last 1000 seconds of that UTC day, starting at 23:43:19, the UTC-
   SLS clock accelerates to 1.001 times its normal rate, which means
   that the UTC-SLS clock progresses during that time through 1001 "fast
   seconds", each of which lasts 1000/1001 seconds.

   The following table shows the exact and simultaneous display of a UTC
   and UTC-SLS clock at various points in time near a deleted leap

          UTC           UTC-SLS

     23:43:18.0000   23:43:18.0000
     23:43:19.0000   23:43:19.0000   UTC-SLS starts to diverge from UTC
     23:43:20.0000   23:43:20.0010   UTC-SLS is now 1 ms ahead of UTC
     23:43:21.0000   23:43:21.0020   UTC-SLS is now 2 ms ahead of UTC
     23:43:22.0000   23:43:22.0030   UTC-SLS is now 3 ms ahead of UTC
     ... 995 seconds later ...
     23:59:57.0000   23:59:57.9980   UTC-SLS is now 998 ms ahead of UTC
     23:59:58.0000   23:59:58.9990   UTC-SLS is now 999 ms ahead of UTC
     00:00:00.0000   00:00:00.0000   leap second skipped, UTC-SLS = UTC
     00:00:01.0000   00:00:01.0000

   Intermediate values are again defined through linear interpolation,
   for example:

          UTC           UTC-SLS

     23:43:19.1000   23:43:19.1001
     23:43:19.2000   23:43:19.2002

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     23:59:58.9000   23:59:59.8999

5  Conversion between UTC and UTC-SLS

   UTC and UTC-SLS clock displays (of the form hh:mm:ss.sss...) can
   unambiguously be converted into each other, as long as one knows
   whether there will be a leap second within 1000 seconds after the
   time to be converted, and if so, what its sign is.

   For a given time of day written as hh:mm:ss (hh < 24 and mm < 60),
   let "since midnight" denote the value hh * 3600 s + mm * 60 s + ss *
   1 s.  Let U denote the "since midnight" value of a UTC clock display
   and let US denote the corresponding "since midnight" value of a UTC-
   SLS clock display.  Let

     L = +1 s   if the UTC day ends with an inserted leap second,
     L = -1 s   if the UTC day ends with a deleted leap second, and
     L =  0 s   otherwise.

   Further, let

     B = 86400 s + L - I

   where I = 1000 s is the length of the smoothing interval.

   If U < B, then U = US, otherwise

     US = U - L * (U - B) / I


     U  = B + (US - B) / (1 - L / I).

6  Security Considerations

   Leap seconds are only one of several likely reasons due to which an
   application may experience disruptions in the operation of a UTC-
   synchronized clock.  An important other one is the temporary loss of
   synchronization with UTC, for example due to interrupted
   communication channels or an operator error, and the need for
   subsequent resynchronization with UTC.  Most security-sensitive
   systems cannot afford to rely on an assumption that their clock is
   always synchronized to UTC with less than +/- 1000 ms error, or that
   the rate of their clock does not deviate by up to +/- 0.1% when
   measured over intervals less than 1000 seconds long.  It is,
   therefore, unlikely that the use of UTC-SLS is going to introduce any
   new security hazards into an application that would not exist without
   UTC-SLS and that are not due to unrealistic expectations in the

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   performance of any computer-implemented UTC-synchronized clock.

APPENDIX A: Rationale

   This section lists some of the options considered during the
   development of this specification, and indicates the reasons for the
   particular design chosen for UTC-SLS.

   Functions that read a UTC-synchronized clock and return the current
   time as a scalar value could behave in a number of different ways
   near UTC leap seconds.  If the scalar timescale allocates an interval
   of 86400 seconds for each UTC day and the goal is not to deviate in
   any way from UTC outside a leap second, possible behaviors during
   inserted UTC leap second include:

   1a) The scalar value could jump backwards in time by one second at
       the start of an inserted leap second (so, for example, both
       23:59:59.1 and 23:59:60.1 will have the same scalar
       representation, and the last second of the day repeats).

   1b) The scalar value could jump backwards in time by one second at
       the end of an inserted leap second (so, for example, both
       23:59:60.1 and 00:00:00.1 will have the same scalar
       representation, and the first second of the next day repeats).

   1c) The function could return, in addition, a leap-second-in-progress
       bit, to disambiguate the scalar value.

   1d) Where a separate second and subsecond value is returned, an
       unambiguous out-of-range subsecond component could be returned,
       until the inserted leap second is over (for example, a nanosecond
       count could overflow in the range 1000000000 to 1999999999).

   1e) The function could delay its reading of the clock and not return
       until the inserted leap second is over.

   1f) The clock could stop for 1 s right before reaching 00:00:00.

   1g) The operating system could suspend all processes during an
       inserted leap second.

   1h) The function could abort with an error code when called during an
       inserted leap second.

   Deleted leap seconds leave less room for variety and a requirement
   that the returned value must not deviate in any way from UTC outside
   a leap second could only be achieved in one way:

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   1i) The scalar value jumps forward in time by one second when the
       deleted leap second 23:59:59 is reached, directly to 00:00:00.

   Other options include:

   2a) The scalar timescale could be defined by allocating 86401 seconds
       for each UTC day.  This would provide an unambiguous scalar
       representation of an inserted leap second 23:59:60 at the end of
       each day.

   2b) The scalar timescale could be defined by allocating the actual
       number of seconds to each day, this way representing a count of
       real seconds, rather than being a fixed encoding of a YYYY-MM-DD
       hh:mm:ss UTC clock reading.  The conversion between such a scalar
       time and a vector representation of UTC would then dependent on
       the availability of a table of all leap seconds since the origin
       of the scale ("epoch").

   2c) The clock could continue without any adjustment across a leap
       second, delaying the leap until the system is restarted.

   2d) The scalar timescale could be defined by allocating 86400 seconds
       for each UTC day, but the rate at which the clock ticks through
       the seconds could be changed temporarily.

   Options 1a) to 1i) and also 2a) all lead to discontinuities in the
   time scale.  If an application measures the time interval between two
   events by subtracting two of the returned values, then with any of
   these options, the relative error made has no upper bound.

   The problem with option 2b) is that the mapping between scalar time
   values and integer vectors representing the display of a UTC clock is
   no longer fixed.  This mapping changes each time a new leap second is
   announced and is not predictable more than a few months into the
   future.  So while option 2b) may be very attractive for applications
   with high accuracy requirements for time-interval measurements (e.g.,
   navigating a spacecraft), it is not well suited for applications that
   rely on a stable future relationship with UTC (e.g., an office

   Option 2c) avoids clock discontinuities while processes are running
   on the local system.  However, it may hinder interoperability with
   systems that restart at different times.

   While [C] permits all of the above approaches, [POSIX] explicitely
   forbids both 2a) and 2b) for its "seconds since the epoch" timescale.

   This leaves option 2d), the solution chosen in this specification, as

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   the one that is easiest to manage and least likely to cause
   disruption for a large number of applications.  The form of the
   required rate correction can be chosen in a number of ways, for

   3a) The clock rate switches between three values: normal, slow, fast.

   3b) The clock rate is ramped up linearly from its normal rate to a
       peak deviation, and then ramped back again to normal.

   3c) The difference in offset of the timescale before and after the
       leap second is fed into the same control loop that keeps the
       offset and rate of the clock aligned with an external reference.
       The one-second step will be processed by the loop's low-pass
       filters, whose output gradually adjusts the rate and offset of
       the clock for a smooth transition.

   Option 3a) was chosen for UTC-SLS, because it is the easiest of these
   alternatives to describe, understand, implement and verify.  With it,
   the mapping between a fixed-frequency counter value and a scalar
   representation of UTC-SLS is a continuous function that is piece-wise
   defined through affine functions (polynomials of degree one).

   With option 3b), that mapping would become a continuously
   differentiable function that is defined piece-wise through
   polynomials of degree two.  This increase in conceptual and
   computational complexity would have the benefit of limiting the rate
   at which the rate of the clock changes.  There may be some specialist
   applications that could benefit from such a limit, in particular
   those where a control loop is used to steer the motion of a large
   mass (e.g., a real or simulated "fly wheel") in relation to a UTC-
   synchronized clock.  Besides the question whether UTC-based time
   scales are appropriate at all for such applications, given that each
   would have its own particular engineering constraints, it seems more
   appropriate to use a special-purpose timescale for each, rather than
   attempt to try and accommodate them all in a general-purpose solution
   like UTC-SLS.

   Option 3c) may seem easiest to implement with an already existing
   control loop.  However, there are a large number of design choices
   and parameters with such control loops, which designers may want to
   optimize based on the properties of their particular clock hardware
   and communication channel.  Therefore, standardizing any particular
   one control loop for the purpose of smoothing leap seconds would
   either place a severe restriction on the designer of a control loop
   that is primarily needed to track the reference clock, or it would
   lead to the implementation of separate control loops, defeating the
   only advantage of option 3c).

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   Having decided to simply switch from the clock's normal rate to a
   single faster or slower smoothing rate near a leap second, two more
   choices need to be made.

   The time interval during which the clock runs at the smoothing rate
   could be placed

   4a) entirely after the leap second;

   4b) entirely before the leap second;

   4c) centered around the leap second.

   Option 4c) would have the advantage that the maximum deviation
   between UTC and UTC-SLS is limited to only half a second.  However,
   this maximum deviation would be reached at midnight, which is a time
   commonly used to schedule events and deadlines.  Both options 4a) and
   4b) lead to a maximum deviation between UTC and UTC-SLS of one
   second, but with them, UTC and UTC-SLS are identical at midnight.
   For this reason, option 4c) was not chosen.

   Many time-signal providers transmit a leap-second announcement during
   some time before the leap second, and stop doing so as soon as the
   leap second is over.  With option 4a), a UTC time-signal receiver
   that is switched on directly after a leap second would not be able to
   learn about the leap second that has just happened, and would,
   therefore, not be able to apply the correction necessary to convert
   UTC into UTC-SLS.  With option 4b), the time during which UTC and
   UTC-SLS differ and the time during which leap-second announcements
   are usually transmitted overlaps, ensuring that a newly activated UTC
   receiver can very quickly synchronize with UTC-SLS.

   The final parameter to be chosen is the length I of the smoothing
   interval, which also defines the factor 1 - L/I by which the clock
   rate changes.  The chosen value I = 1000 s (or 16 minutes and 40
   seconds) fulfills the following requirements:

   5a) The resulting maximal rate error of L/I should be well below any
       human perception limit for changes in length, duration, rate,
       rhythm, or pitch, to make it unlikely that anyone will directly
       perceive the rate change with unaided senses.

   5b) The length of I should be below half an hour (I < 1800 s), such
       that UTC and UTC-SLS are identical at every half and full hour in
       every time zone that differs from UTC by an integral number of
       full or half hours.  This way, many exact deadlines remain
       unaffected by the difference between UTC and UTC-SLS.

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   5c) The length I should be less than 59 minutes, which is the advance
       warning time given by some existing time services for an upcoming
       leap second (e.g., the German DCF77 transmitter).

   5d) Choosing a power of 10 times one second for the value of I
       ensures that the conversion between UTC and UTC-SLS is easy to
       understand and perform when both times are displayed as decimal

   The choice of I = 1000 s is the largest value that fulfills all of
   the above criteria.

APPENDIX B: Application Limits

   Where applications use a UTC-SLS clock to measure the time interval
   between two events, and do not correct for the variable rate, the
   maximum possible relative error due to leap seconds is 0.1%.  This
   maximum error can only be reached for intervals of 1000 s or shorter,
   and decreases for longer intervals.

   The vast majority of computer applications have far less exacting
   requirements for time-interval measurements and can, therefore, use
   UTC-SLS without any concern for leap seconds.

   Some multimedia standards specify stricter clock-stability
   requirements, which cannot be met within the constraints listed in
   Appendix A.  For example, [MPEG] defines a 27 MHz system clock
   frequency with a maximum permitted frequency error of 0.003% (30 ppm,
   parts per million) and a maximum permitted rate change with time of
   75 millihertz per second or 0.002777 ppm/s.

   Whether UTC-SLS can still be used with such specifications depends on
   the requirements of a particular application.  Strict clock-rate
   tolerances, like those given in [MPEG], can be critical in tightly
   synchronized television-studio networks.  On the other hand,
   requirements may be far more relaxed if the same audio-visual data is
   streaming over the Internet.  There, the receiver must buffer data
   anyway for several seconds to compensate network jitter, and a leap
   second smoothed over 1000 seconds becomes negligible compared to the
   normal variability in network delay.

   Examples for other applications with strict clock-stability
   requirements include spacecraft navigation systems, where the motion
   of bodies is measured and predicted far more accurately than with
   0.1% error, or the control of distributed scientific instruments that
   operate in a global reference frame, such as telescopes and
   seismographs.  Such time-critical applications are usually
   implemented on special real-time hardware and software that reduce

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   the many scheduling and timing hazards of general-purpose platforms,
   such as operating systems with preemptive multitasking.  Some real-
   time programming environments provide several clocks with different
   stability properties.  For example, in [POSIX], the clock_gettime()
   function distinguishes between a "REALTIME" and a "MONOTONIC" clock.
   The former is meant to approximate UTC, and can do so in the form of
   UTC-SLS.  The "MONOTONIC" clock, on the other hand, does not
   approximate any external clock, but aims to be as rate stable as
   possible.  It may just count the seconds since the last system
   restart.  It is, therefore, a more suitable choice for sensitive
   time-interval measurements.

   A number of standard time scales exist that are, in contrast to UTC
   and UTC-SLS, not synchronized with Earth's rotation.  They are
   entirely defined by precision clocks and feature no leap seconds.
   UTC falls behind these timescales by one more second with each
   positive leap second.  Some examples are:

     - International Atomic Time (TAI) is a leap-second free timescale
       defined by the same clock network that determines UTC.  It was
       approximately identical to Universal Time on 1958-01-01 and
       was exactly 10 seconds ahead of UTC by 1972-01-01.

     - GPS Time is a timescale used within the U.S. Department of
       Defense Global Positioning System.  It has its origin at
       1980-01-06 00:00 UTC, when TAI was 19 s ahead of UTC.  Therefore,
       GPS Time remains 19 s behind TAI.  (Many GPS receivers can
       display both GPS Time and UTC.)

     - Terrestrial Time is an International Astronomical Union timescale
       used in astronomical tables.  It is 32.184 s ahead of TAI.

   It can be expected that some future operating systems will maintain
   an additional clock that is synchronized with one of these
   timescales.  Such clocks are well suited for highly accurate long-
   term time-interval measurements.  However, they lack the close
   relationship to legal time that UTC-synchronized clocks offer.  And
   because a clock synchronized to TAI or GPS Time may still need
   substantial readjustment after a temporary loss of synchronization,
   they may not guarantee the same short-term rate stability as a clock
   like "MONOTONIC" that is not externally synchronized.

Normative References

    [ITU-460] "Standard-frequency and time-signal emissions", ITU-R
              Recommendation TF.460-6, International Telecommunication
              Union, Geneva, 2002.

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     [IERS-C] Gambis, D., "Bulletin C", International Earth Rotation and
              Reference Systems Service (IERS), Paris, France.

Informal References

      [Nel01] Nelson, R., et al., "The leap second: its history and
              possible future", Metrologia, Vol. 38, 2001, pp. 509-529.

      [McC91] McCarthy, D.: "Astronomical Time", Proceedings of the
              IEEE, Vol. 79, No. 7, July 1991, pp. 915-920.

    [ITU-768] "Standard frequencies and time signals", ITU-R
              Recommendation TF.768-5, International Telecommunication
              Union, Geneva, 2002.

    [ISO8601] "Data elements and interchange formats -- Information
              interchange -- Representation of dates and times", ISO
              8601, International Organization for Standardization,
              Geneva, 2004.

    [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, July 2002.

    [RFC1305] D. Mills, "Network Time Protocol (Version 3) :
              Specification, Implementation and Analysis", RFC 1305,
              March 1992.

    [IEC1588] "Precision clock synchronization protocol for networked
              measurement and control systems", IEC 61588, International
              Electrotechnical Commission, Geneva, 2004.

      [POSIX] The Open Group, "Single UNIX Specification", Version 3,
              Base Specifications Issue 6, IEEE Std 1003.1, 2004
              edition.  http://www.unix.org/

          [C] "Programming languages -- C", ISO/IEC 9899, International
              Organization for Standardization, Geneva, 1999.

       [MPEG] "Information technology -- Generic coding of moving
              pictures and associated audio information -- Part 1:
              Systems", ISO/IEC 13818-1, 1997.

Author's Address

   Markus G. Kuhn
   University of Cambridge
   Computer Laboratory

Kuhn                                                          [Page 14]

INTERNET-DRAFT                   UTC-SLS                    January 2006

   15 JJ Thomson Avenue
   Cambridge CB3 0FD

   Email: Markus.Kuhn@cl.cam.ac.uk
   Phone: +44 1223 334676
   WWW:   http://www.cl.cam.ac.uk/~mgk25/

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