leap-seconds.list revision 280414
1275970Scy# 2275970Scy# In the following text, the symbol '#' introduces 3275970Scy# a comment, which continues from that symbol until 4275970Scy# the end of the line. A plain comment line has a 5275970Scy# whitespace character following the comment indicator. 6275970Scy# There are also special comment lines defined below. 7275970Scy# A special comment will always have a non-whitespace 8275970Scy# character in column 2. 9275970Scy# 10275970Scy# A blank line should be ignored. 11275970Scy# 12275970Scy# The following table shows the corrections that must 13275970Scy# be applied to compute International Atomic Time (TAI) 14275970Scy# from the Coordinated Universal Time (UTC) values that 15275970Scy# are transmitted by almost all time services. 16275970Scy# 17275970Scy# The first column shows an epoch as a number of seconds 18275970Scy# since 1 January 1900, 00:00:00 (1900.0 is also used to 19275970Scy# indicate the same epoch.) Both of these time stamp formats 20275970Scy# ignore the complexities of the time scales that were 21275970Scy# used before the current definition of UTC at the start 22275970Scy# of 1972. (See note 3 below.) 23275970Scy# The second column shows the number of seconds that 24275970Scy# must be added to UTC to compute TAI for any timestamp 25275970Scy# at or after that epoch. The value on each line is 26275970Scy# valid from the indicated initial instant until the 27275970Scy# epoch given on the next one or indefinitely into the 28275970Scy# future if there is no next line. 29275970Scy# (The comment on each line shows the representation of 30275970Scy# the corresponding initial epoch in the usual 31275970Scy# day-month-year format. The epoch always begins at 32275970Scy# 00:00:00 UTC on the indicated day. See Note 5 below.) 33275970Scy# 34275970Scy# Important notes: 35275970Scy# 36275970Scy# 1. Coordinated Universal Time (UTC) is often referred to 37275970Scy# as Greenwich Mean Time (GMT). The GMT time scale is no 38275970Scy# longer used, and the use of GMT to designate UTC is 39275970Scy# discouraged. 40275970Scy# 41275970Scy# 2. The UTC time scale is realized by many national 42275970Scy# laboratories and timing centers. Each laboratory 43275970Scy# identifies its realization with its name: Thus 44275970Scy# UTC(NIST), UTC(USNO), etc. The differences among 45275970Scy# these different realizations are typically on the 46275970Scy# order of a few nanoseconds (i.e., 0.000 000 00x s) 47275970Scy# and can be ignored for many purposes. These differences 48275970Scy# are tabulated in Circular T, which is published monthly 49275970Scy# by the International Bureau of Weights and Measures 50275970Scy# (BIPM). See www.bipm.org for more information. 51275970Scy# 52275970Scy# 3. The current definition of the relationship between UTC 53275970Scy# and TAI dates from 1 January 1972. A number of different 54275970Scy# time scales were in use before that epoch, and it can be 55275970Scy# quite difficult to compute precise timestamps and time 56275970Scy# intervals in those "prehistoric" days. For more information, 57275970Scy# consult: 58275970Scy# 59275970Scy# The Explanatory Supplement to the Astronomical 60275970Scy# Ephemeris. 61275970Scy# or 62275970Scy# Terry Quinn, "The BIPM and the Accurate Measurement 63275970Scy# of Time," Proc. of the IEEE, Vol. 79, pp. 894-905, 64275970Scy# July, 1991. 65275970Scy# 66275970Scy# 4. The decision to insert a leap second into UTC is currently 67275970Scy# the responsibility of the International Earth Rotation and 68275970Scy# Reference Systems Service. (The name was changed from the 69275970Scy# International Earth Rotation Service, but the acronym IERS 70275970Scy# is still used.) 71275970Scy# 72275970Scy# Leap seconds are announced by the IERS in its Bulletin C. 73275970Scy# 74275970Scy# See www.iers.org for more details. 75275970Scy# 76275970Scy# Every national laboratory and timing center uses the 77275970Scy# data from the BIPM and the IERS to construct UTC(lab), 78275970Scy# their local realization of UTC. 79275970Scy# 80275970Scy# Although the definition also includes the possibility 81275970Scy# of dropping seconds ("negative" leap seconds), this has 82275970Scy# never been done and is unlikely to be necessary in the 83275970Scy# foreseeable future. 84275970Scy# 85275970Scy# 5. If your system keeps time as the number of seconds since 86275970Scy# some epoch (e.g., NTP timestamps), then the algorithm for 87275970Scy# assigning a UTC time stamp to an event that happens during a positive 88275970Scy# leap second is not well defined. The official name of that leap 89275970Scy# second is 23:59:60, but there is no way of representing that time 90275970Scy# in these systems. 91275970Scy# Many systems of this type effectively stop the system clock for 92275970Scy# one second during the leap second and use a time that is equivalent 93275970Scy# to 23:59:59 UTC twice. For these systems, the corresponding TAI 94275970Scy# timestamp would be obtained by advancing to the next entry in the 95275970Scy# following table when the time equivalent to 23:59:59 UTC 96275970Scy# is used for the second time. Thus the leap second which 97275970Scy# occurred on 30 June 1972 at 23:59:59 UTC would have TAI 98275970Scy# timestamps computed as follows: 99275970Scy# 100275970Scy# ... 101275970Scy# 30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds 102275970Scy# 30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds 103275970Scy# 1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds 104275970Scy# ... 105275970Scy# 106275970Scy# If your system realizes the leap second by repeating 00:00:00 UTC twice 107275970Scy# (this is possible but not usual), then the advance to the next entry 108275970Scy# in the table must occur the second time that a time equivalent to 109275970Scy# 00:00:00 UTC is used. Thus, using the same example as above: 110275970Scy# 111275970Scy# ... 112275970Scy# 30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds 113275970Scy# 30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds 114275970Scy# 1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds 115275970Scy# ... 116275970Scy# 117275970Scy# in both cases the use of timestamps based on TAI produces a smooth 118275970Scy# time scale with no discontinuity in the time interval. However, 119275970Scy# although the long-term behavior of the time scale is correct in both 120275970Scy# methods, the second method is technically not correct because it adds 121275970Scy# the extra second to the wrong day. 122275970Scy# 123275970Scy# This complexity would not be needed for negative leap seconds (if they 124275970Scy# are ever used). The UTC time would skip 23:59:59 and advance from 125275970Scy# 23:59:58 to 00:00:00 in that case. The TAI offset would decrease by 126275970Scy# 1 second at the same instant. This is a much easier situation to deal 127275970Scy# with, since the difficulty of unambiguously representing the epoch 128275970Scy# during the leap second does not arise. 129275970Scy# 130275970Scy# Some systems implement leap seconds by amortizing the leap second 131275970Scy# over the last few minutes of the day. The frequency of the local 132275970Scy# clock is decreased (or increased) to realize the positive (or 133275970Scy# negative) leap second. This method removes the time step described 134275970Scy# above. Although the long-term behavior of the time scale is correct 135275970Scy# in this case, this method introduces an error during the adjustment 136275970Scy# period both in time and in frequency with respect to the official 137275970Scy# definition of UTC. 138275970Scy# 139275970Scy# Questions or comments to: 140275970Scy# Judah Levine 141289997Sglebius# Time and Frequency Division 142289997Sglebius# NIST 143275970Scy# Boulder, Colorado 144275970Scy# Judah.Levine@nist.gov 145275970Scy# 146275970Scy# Last Update of leap second values: 5 January 2015 147275970Scy# 148275970Scy# The following line shows this last update date in NTP timestamp 149275970Scy# format. This is the date on which the most recent change to 150275970Scy# the leap second data was added to the file. This line can 151275970Scy# be identified by the unique pair of characters in the first two 152275970Scy# columns as shown below. 153275970Scy# 154275970Scy#$ 3629404800 155275970Scy# 156275970Scy# The NTP timestamps are in units of seconds since the NTP epoch, 157275970Scy# which is 1 January 1900, 00:00:00. The Modified Julian Day number 158275970Scy# corresponding to the NTP time stamp, X, can be computed as 159275970Scy# 160275970Scy# X/86400 + 15020 161275970Scy# 162275970Scy# where the first term converts seconds to days and the second 163275970Scy# term adds the MJD corresponding to the time origin defined above. 164275970Scy# The integer portion of the result is the integer MJD for that 165275970Scy# day, and any remainder is the time of day, expressed as the 166275970Scy# fraction of the day since 0 hours UTC. The conversion from day 167275970Scy# fraction to seconds or to hours, minutes, and seconds may involve 168275970Scy# rounding or truncation, depending on the method used in the 169275970Scy# computation. 170275970Scy# 171275970Scy# The data in this file will be updated periodically as new leap 172275970Scy# seconds are announced. In addition to being entered on the line 173275970Scy# above, the update time (in NTP format) will be added to the basic 174275970Scy# file name leap-seconds to form the name leap-seconds.<NTP TIME>. 175275970Scy# In addition, the generic name leap-seconds.list will always point to 176275970Scy# the most recent version of the file. 177275970Scy# 178275970Scy# This update procedure will be performed only when a new leap second 179275970Scy# is announced. 180275970Scy# 181275970Scy# The following entry specifies the expiration date of the data 182275970Scy# in this file in units of seconds since the origin at the instant 183275970Scy# 1 January 1900, 00:00:00. This expiration date will be changed 184275970Scy# at least twice per year whether or not a new leap second is 185275970Scy# announced. These semi-annual changes will be made no later 186275970Scy# than 1 June and 1 December of each year to indicate what 187275970Scy# action (if any) is to be taken on 30 June and 31 December, 188275970Scy# respectively. (These are the customary effective dates for new 189275970Scy# leap seconds.) This expiration date will be identified by a 190275970Scy# unique pair of characters in columns 1 and 2 as shown below. 191275970Scy# In the unlikely event that a leap second is announced with an 192275970Scy# effective date other than 30 June or 31 December, then this 193275970Scy# file will be edited to include that leap second as soon as it is 194275970Scy# announced or at least one month before the effective date 195275970Scy# (whichever is later). 196275970Scy# If an announcement by the IERS specifies that no leap second is 197275970Scy# scheduled, then only the expiration date of the file will 198275970Scy# be advanced to show that the information in the file is still 199275970Scy# current -- the update time stamp, the data and the name of the file 200275970Scy# will not change. 201275970Scy# 202275970Scy# Updated through IERS Bulletin C49 203275970Scy# File expires on: 28 December 2015 204275970Scy# 205275970Scy#@ 3660249600 206# 2072272060800 10 # 1 Jan 1972 2082287785600 11 # 1 Jul 1972 2092303683200 12 # 1 Jan 1973 2102335219200 13 # 1 Jan 1974 2112366755200 14 # 1 Jan 1975 2122398291200 15 # 1 Jan 1976 2132429913600 16 # 1 Jan 1977 2142461449600 17 # 1 Jan 1978 2152492985600 18 # 1 Jan 1979 2162524521600 19 # 1 Jan 1980 2172571782400 20 # 1 Jul 1981 2182603318400 21 # 1 Jul 1982 2192634854400 22 # 1 Jul 1983 2202698012800 23 # 1 Jul 1985 2212776982400 24 # 1 Jan 1988 2222840140800 25 # 1 Jan 1990 2232871676800 26 # 1 Jan 1991 2242918937600 27 # 1 Jul 1992 2252950473600 28 # 1 Jul 1993 2262982009600 29 # 1 Jul 1994 2273029443200 30 # 1 Jan 1996 2283076704000 31 # 1 Jul 1997 2293124137600 32 # 1 Jan 1999 2303345062400 33 # 1 Jan 2006 2313439756800 34 # 1 Jan 2009 2323550089600 35 # 1 Jul 2012 2333644697600 36 # 1 Jul 2015 234# 235# the following special comment contains the 236# hash value of the data in this file computed 237# use the secure hash algorithm as specified 238# by FIPS 180-1. See the files in ~/pub/sha for 239# the details of how this hash value is 240# computed. Note that the hash computation 241# ignores comments and whitespace characters 242# in data lines. It includes the NTP values 243# of both the last modification time and the 244# expiration time of the file, but not the 245# white space on those lines. 246# the hash line is also ignored in the 247# computation. 248# 249#h 45e70fa7 a9df2033 f4a49ab0 ec648273 7b6c22c 250