Edit File: leap-seconds.list
# # 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 1 January 1900, 00:00:00 (1900.0 is also used to # indicate the same epoch.) Both of these time stamp formats # ignore the complexities of the time scales that were # used before the current definition of UTC at the start # of 1972. (See note 3 below.) # 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.org 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 that 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. <http://dx.doi.org/10.1109/5.84965> # reprinted in: # Christine Hackman and Donald B Sullivan (eds.) # Time and Frequency Measurement # American Association of Physics Teachers (1996) # <http://tf.nist.gov/general/pdf/1168.pdf>, pp. 75-86 # # 4. The decision to insert a leap second into UTC is currently # the responsibility of the International Earth Rotation and # Reference Systems Service. (The name was changed from the # International Earth Rotation Service, but the acronym IERS # is still used.) # # Leap seconds are announced by the IERS in its Bulletin C. # # See www.iers.org for more details. # # Every national laboratory and timing center uses the # data from the BIPM and the IERS to construct UTC(lab), # their local realization 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 equivalent 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. However, # although the long-term behavior of the time scale is correct in both # methods, the second method is technically not correct because it adds # the extra second to the wrong day. # # 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. # # Some systems implement leap seconds by amortizing the leap second # over the last few minutes of the day. The frequency of the local # clock is decreased (or increased) to realize the positive (or # negative) leap second. This method removes the time step described # above. Although the long-term behavior of the time scale is correct # in this case, this method introduces an error during the adjustment # period both in time and in frequency with respect to the official # definition of UTC. # # Questions or comments to: # Judah Levine # Time and Frequency Division # NIST # Boulder, Colorado # Judah.Levine@nist.gov # # Last Update of leap second values: 8 July 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. # #$ 3676924800 # # The NTP timestamps are in units of seconds since the NTP epoch, # which is 1 January 1900, 00:00:00. The Modified Julian Day number # corresponding to the NTP time stamp, X, can be computed as # # X/86400 + 15020 # # where the first term converts seconds to days and the second # term adds the MJD corresponding to the time origin defined above. # The integer portion of the result is the integer MJD for that # day, and any remainder is the time of day, expressed as the # fraction of the day since 0 hours UTC. The conversion from day # fraction to seconds or to hours, minutes, and seconds may involve # rounding or truncation, depending on the method used in the # computation. # # 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 the origin at the instant # 1 January 1900, 00:00:00. 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 C65 # File expires on: 28 December 2023 # #@ 3912710400 # 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 ~/pub/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 e76a99dc 65f15cc7 e613e040 f5078b5e b23834fe