MFC r304779, r304780, r304781, r304782, r304802

[FreeBSD/stable/10.git] / etc / ntp / leap-seconds

#
#       In the following text, the symbol '#' introduces
#       a comment, which continues from that symbol until 
#       the end of the line. A plain comment line has a
#       whitespace character following the comment indicator.
#       There are also special comment lines defined below. 
#       A special comment will always have a non-whitespace 
#       character in column 2.
#
#       A blank line should be ignored.
#
#       The following table shows the corrections that must
#       be applied to compute International Atomic Time (TAI)
#       from the Coordinated Universal Time (UTC) values that
#       are transmitted by almost all time services.
#
#       The first column shows an epoch as a number of seconds
#       since 1900.0 and the second column shows the number of
#       seconds that must be added to UTC to compute TAI for
#       any timestamp at or after that epoch. The value on 
#       each line is valid from the indicated initial instant
#       until the epoch given on the next one or indefinitely 
#       into the future if there is no next line.
#       (The comment on each line shows the representation of
#       the corresponding initial epoch in the usual 
#       day-month-year format. The epoch always begins at
#       00:00:00 UTC on the indicated day. See Note 5 below.)
#       
#       Important notes:
#
#       1. Coordinated Universal Time (UTC) is often referred to
#       as Greenwich Mean Time (GMT). The GMT time scale is no
#       longer used, and the use of GMT to designate UTC is
#       discouraged.
#
#       2. The UTC time scale is realized by many national 
#       laboratories and timing centers. Each laboratory
#       identifies its realization with its name: Thus
#       UTC(NIST), UTC(USNO), etc. The differences among
#       these different realizations are typically on the
#       order of a few nanoseconds (i.e., 0.000 000 00x s)
#       and can be ignored for many purposes. These differences
#       are tabulated in Circular T, which is published monthly
#       by the International Bureau of Weights and Measures
#       (BIPM). See www.bipm.fr for more information.
#
#       3. The current defintion of the relationship between UTC 
#       and TAI dates from 1 January 1972. A number of different 
#       time scales were in use before than epoch, and it can be 
#       quite difficult to compute precise timestamps and time 
#       intervals in those "prehistoric" days. For more information,
#       consult:
#
#               The Explanatory Supplement to the Astronomical
#               Ephemeris.
#       or
#               Terry Quinn, "The BIPM and the Accurate Measurement
#               of Time," Proc. of the IEEE, Vol. 79, pp. 894-905,
#               July, 1991.
#
#       4.  The insertion of leap seconds into UTC is currently the
#       responsibility of the International Earth Rotation Service,
#       which is located at the Paris Observatory: 
#
#       Central Bureau of IERS
#       61, Avenue de l'Observatoire
#       75014 Paris, France.
#
#       Leap seconds are announced by the IERS in its Bulletin C
#
#       See hpiers.obspm.fr or www.iers.org for more details.
#
#       All national laboratories and timing centers use the
#       data from the BIPM and the IERS to construct their
#       local realizations of UTC.
#
#       Although the definition also includes the possibility
#       of dropping seconds ("negative" leap seconds), this has 
#       never been done and is unlikely to be necessary in the 
#       foreseeable future.
#
#       5. If your system keeps time as the number of seconds since
#       some epoch (e.g., NTP timestamps), then the algorithm for
#       assigning a UTC time stamp to an event that happens during a positive
#       leap second is not well defined. The official name of that leap 
#       second is 23:59:60, but there is no way of representing that time 
#       in these systems. 
#       Many systems of this type effectively stop the system clock for 
#       one second during the leap second and use a time that is equivalent 
#       to 23:59:59 UTC twice. For these systems, the corresponding TAI 
#       timestamp would be obtained by advancing to the next entry in the
#       following table when the time equivalent to 23:59:59 UTC
#       is used for the second time. Thus the leap second which
#       occurred on 30 June 1972 at 23:59:59 UTC would have TAI
#       timestamps computed as follows:
#
#       ...
#       30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds
#       30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds
#       1  July 1972 00:00:00 (2287785600)              TAI= UTC + 11 seconds
#       ...
#
#       If your system realizes the leap second by repeating 00:00:00 UTC twice
#       (this is possible but not usual), then the advance to the next entry
#       in the table must occur the second time that a time equivlent to 
#       00:00:00 UTC is used. Thus, using the same example as above:
#
#       ...
#       30 June 1972 23:59:59 (2287785599):             TAI= UTC + 10 seconds
#       30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds
#       1  July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds
#       ...
#
#       in both cases the use of timestamps based on TAI produces a smooth
#       time scale with no discontinuity in the time interval.
#
#       This complexity would not be needed for negative leap seconds (if they 
#       are ever used). The UTC time would skip 23:59:59 and advance from 
#       23:59:58 to 00:00:00 in that case.  The TAI offset would decrease by 
#       1 second at the same instant.  This is a much easier situation to deal 
#       with, since the difficulty of unambiguously representing the epoch 
#       during the leap second does not arise.
#
#       Questions or comments to:
#               Jeff Prillaman
#               Time Service Department
#               US Naval Observatory
#               Washington, DC
#               jeffrey.prillaman@usno.navy.mil
#
#       Last Update of leap second values:   6 Jul 2016
#
#       The following line shows this last update date in NTP timestamp 
#       format. This is the date on which the most recent change to
#       the leap second data was added to the file. This line can
#       be identified by the unique pair of characters in the first two 
#       columns as shown below.
#
#$       3676752000
#
#       The data in this file will be updated periodically as new leap 
#       seconds are announced. In addition to being entered on the line
#       above, the update time (in NTP format) will be added to the basic 
#       file name leap-seconds to form the name leap-seconds.<NTP TIME>.
#       In addition, the generic name leap-seconds.list will always point to 
#       the most recent version of the file.
#
#       This update procedure will be performed only when a new leap second
#       is announced. 
#
#       The following entry specifies the expiration date of the data
#       in this file in units of seconds since 1900.0.  This expiration date 
#       will be changed at least twice per year whether or not a new leap 
#       second is announced. These semi-annual changes will be made no
#       later than 1 June and 1 December of each year to indicate what
#       action (if any) is to be taken on 30 June and 31 December, 
#       respectively. (These are the customary effective dates for new
#       leap seconds.) This expiration date will be identified by a
#       unique pair of characters in columns 1 and 2 as shown below.
#       In the unlikely event that a leap second is announced with an 
#       effective date other than 30 June or 31 December, then this
#       file will be edited to include that leap second as soon as it is
#       announced or at least one month before the effective date
#       (whichever is later). 
#       If an announcement by the IERS specifies that no leap second is 
#       scheduled, then only the expiration date of the file will 
#       be advanced to show that the information in the file is still
#       current -- the update time stamp, the data and the name of the file 
#       will not change.
#
#       Updated through IERS Bulletin C 52
#       File expires on:  1 Jun 2017
#
#@      3705264000
#
2272060800      10      # 1 Jan 1972
2287785600      11      # 1 Jul 1972
2303683200      12      # 1 Jan 1973
2335219200      13      # 1 Jan 1974
2366755200      14      # 1 Jan 1975
2398291200      15      # 1 Jan 1976
2429913600      16      # 1 Jan 1977
2461449600      17      # 1 Jan 1978
2492985600      18      # 1 Jan 1979
2524521600      19      # 1 Jan 1980
2571782400      20      # 1 Jul 1981
2603318400      21      # 1 Jul 1982
2634854400      22      # 1 Jul 1983
2698012800      23      # 1 Jul 1985
2776982400      24      # 1 Jan 1988
2840140800      25      # 1 Jan 1990
2871676800      26      # 1 Jan 1991
2918937600      27      # 1 Jul 1992
2950473600      28      # 1 Jul 1993
2982009600      29      # 1 Jul 1994
3029443200      30      # 1 Jan 1996
3076704000      31      # 1 Jul 1997
3124137600      32      # 1 Jan 1999
3345062400      33      # 1 Jan 2006
3439756800      34      # 1 Jan 2009
3550089600      35      # 1 Jul 2012
3644697600      36      # 1 Jul 2015
3692217600      37      # 1 Jan 2017
#
#       the following special comment contains the
#       hash value of the data in this file computed
#       use the secure hash algorithm as specified
#       by FIPS 180-1. See the files in ~/sha for
#       the details of how this hash value is
#       computed. Note that the hash computation
#       ignores comments and whitespace characters
#       in data lines. It includes the NTP values
#       of both the last modification time and the 
#       expiration time of the file, but not the
#       white space on those lines.
#       the hash line is also ignored in the
#       computation.
#
#h      63f8fea8 587c099d abcf130a ad525eae 3e105052
#