# # $FreeBSD: releng/11.0/etc/ntp/leap-seconds 303407 2016-07-27 19:39:25Z cy $ # # 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 definition 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.. # 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 #