1# 2# In the following text, the symbol '#' introduces 3# a comment, which continues from that symbol until 4# the end of the line. A plain comment line has a 5# whitespace character following the comment indicator. 6# There are also special comment lines defined below. 7# A special comment will always have a non-whitespace 8# character in column 2. 9# 10# A blank line should be ignored. 11# 12# The following table shows the corrections that must 13# be applied to compute International Atomic Time (TAI) 14# from the Coordinated Universal Time (UTC) values that 15# are transmitted by almost all time services. 16# 17# The first column shows an epoch as a number of seconds 18# since 1 January 1900, 00:00:00 (1900.0 is also used to 19# indicate the same epoch.) Both of these time stamp formats 20# ignore the complexities of the time scales that were 21# used before the current definition of UTC at the start 22# of 1972. (See note 3 below.) 23# The second column shows the number of seconds that 24# must be added to UTC to compute TAI for any timestamp 25# at or after that epoch. The value on each line is 26# valid from the indicated initial instant until the 27# epoch given on the next one or indefinitely into the 28# future if there is no next line. 29# (The comment on each line shows the representation of 30# the corresponding initial epoch in the usual 31# day-month-year format. The epoch always begins at 32# 00:00:00 UTC on the indicated day. See Note 5 below.) 33# 34# Important notes: 35# 36# 1. Coordinated Universal Time (UTC) is often referred to 37# as Greenwich Mean Time (GMT). The GMT time scale is no 38# longer used, and the use of GMT to designate UTC is 39# discouraged. 40# 41# 2. The UTC time scale is realized by many national 42# laboratories and timing centers. Each laboratory 43# identifies its realization with its name: Thus 44# UTC(NIST), UTC(USNO), etc. The differences among 45# these different realizations are typically on the 46# order of a few nanoseconds (i.e., 0.000 000 00x s) 47# and can be ignored for many purposes. These differences 48# are tabulated in Circular T, which is published monthly 49# by the International Bureau of Weights and Measures 50# (BIPM). See www.bipm.org for more information. 51# 52# 3. The current definition of the relationship between UTC 53# and TAI dates from 1 January 1972. A number of different 54# time scales were in use before that epoch, and it can be 55# quite difficult to compute precise timestamps and time 56# intervals in those "prehistoric" days. For more information, 57# consult: 58# 59# The Explanatory Supplement to the Astronomical 60# Ephemeris. 61# or 62# Terry Quinn, "The BIPM and the Accurate Measurement 63# of Time," Proc. of the IEEE, Vol. 79, pp. 894-905, 64# July, 1991. <http://dx.doi.org/10.1109/5.84965> 65# reprinted in: 66# Christine Hackman and Donald B Sullivan (eds.) 67# Time and Frequency Measurement 68# American Association of Physics Teachers (1996) 69# <http://tf.nist.gov/general/pdf/1168.pdf>, pp. 75-86 70# 71# 4. The decision to insert a leap second into UTC is currently 72# the responsibility of the International Earth Rotation and 73# Reference Systems Service. (The name was changed from the 74# International Earth Rotation Service, but the acronym IERS 75# is still used.) 76# 77# Leap seconds are announced by the IERS in its Bulletin C. 78# 79# See www.iers.org for more details. 80# 81# Every national laboratory and timing center uses the 82# data from the BIPM and the IERS to construct UTC(lab), 83# their local realization of UTC. 84# 85# Although the definition also includes the possibility 86# of dropping seconds ("negative" leap seconds), this has 87# never been done and is unlikely to be necessary in the 88# foreseeable future. 89# 90# 5. If your system keeps time as the number of seconds since 91# some epoch (e.g., NTP timestamps), then the algorithm for 92# assigning a UTC time stamp to an event that happens during a positive 93# leap second is not well defined. The official name of that leap 94# second is 23:59:60, but there is no way of representing that time 95# in these systems. 96# Many systems of this type effectively stop the system clock for 97# one second during the leap second and use a time that is equivalent 98# to 23:59:59 UTC twice. For these systems, the corresponding TAI 99# timestamp would be obtained by advancing to the next entry in the 100# following table when the time equivalent to 23:59:59 UTC 101# is used for the second time. Thus the leap second which 102# occurred on 30 June 1972 at 23:59:59 UTC would have TAI 103# timestamps computed as follows: 104# 105# ... 106# 30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds 107# 30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds 108# 1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds 109# ... 110# 111# If your system realizes the leap second by repeating 00:00:00 UTC twice 112# (this is possible but not usual), then the advance to the next entry 113# in the table must occur the second time that a time equivalent to 114# 00:00:00 UTC is used. Thus, using the same example as above: 115# 116# ... 117# 30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds 118# 30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds 119# 1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds 120# ... 121# 122# in both cases the use of timestamps based on TAI produces a smooth 123# time scale with no discontinuity in the time interval. However, 124# although the long-term behavior of the time scale is correct in both 125# methods, the second method is technically not correct because it adds 126# the extra second to the wrong day. 127# 128# This complexity would not be needed for negative leap seconds (if they 129# are ever used). The UTC time would skip 23:59:59 and advance from 130# 23:59:58 to 00:00:00 in that case. The TAI offset would decrease by 131# 1 second at the same instant. This is a much easier situation to deal 132# with, since the difficulty of unambiguously representing the epoch 133# during the leap second does not arise. 134# 135# Some systems implement leap seconds by amortizing the leap second 136# over the last few minutes of the day. The frequency of the local 137# clock is decreased (or increased) to realize the positive (or 138# negative) leap second. This method removes the time step described 139# above. Although the long-term behavior of the time scale is correct 140# in this case, this method introduces an error during the adjustment 141# period both in time and in frequency with respect to the official 142# definition of UTC. 143# 144# Questions or comments to: 145# Judah Levine 146# Time and Frequency Division 147# NIST 148# Boulder, Colorado 149# Judah.Levine@nist.gov 150# 151# Last Update of leap second values: 8 July 2016 152# 153# The following line shows this last update date in NTP timestamp 154# format. This is the date on which the most recent change to 155# the leap second data was added to the file. This line can 156# be identified by the unique pair of characters in the first two 157# columns as shown below. 158# 159#$ 3676924800 160# 161# The NTP timestamps are in units of seconds since the NTP epoch, 162# which is 1 January 1900, 00:00:00. The Modified Julian Day number 163# corresponding to the NTP time stamp, X, can be computed as 164# 165# X/86400 + 15020 166# 167# where the first term converts seconds to days and the second 168# term adds the MJD corresponding to the time origin defined above. 169# The integer portion of the result is the integer MJD for that 170# day, and any remainder is the time of day, expressed as the 171# fraction of the day since 0 hours UTC. The conversion from day 172# fraction to seconds or to hours, minutes, and seconds may involve 173# rounding or truncation, depending on the method used in the 174# computation. 175# 176# The data in this file will be updated periodically as new leap 177# seconds are announced. In addition to being entered on the line 178# above, the update time (in NTP format) will be added to the basic 179# file name leap-seconds to form the name leap-seconds.<NTP TIME>. 180# In addition, the generic name leap-seconds.list will always point to 181# the most recent version of the file. 182# 183# This update procedure will be performed only when a new leap second 184# is announced. 185# 186# The following entry specifies the expiration date of the data 187# in this file in units of seconds since the origin at the instant 188# 1 January 1900, 00:00:00. This expiration date will be changed 189# at least twice per year whether or not a new leap second is 190# announced. These semi-annual changes will be made no later 191# than 1 June and 1 December of each year to indicate what 192# action (if any) is to be taken on 30 June and 31 December, 193# respectively. (These are the customary effective dates for new 194# leap seconds.) This expiration date will be identified by a 195# unique pair of characters in columns 1 and 2 as shown below. 196# In the unlikely event that a leap second is announced with an 197# effective date other than 30 June or 31 December, then this 198# file will be edited to include that leap second as soon as it is 199# announced or at least one month before the effective date 200# (whichever is later). 201# If an announcement by the IERS specifies that no leap second is 202# scheduled, then only the expiration date of the file will 203# be advanced to show that the information in the file is still 204# current -- the update time stamp, the data and the name of the file 205# will not change. 206# 207# Updated through IERS Bulletin C62 208# File expires on: 28 June 2022 209# 210#@ 3865363200 211# 2122272060800 10 # 1 Jan 1972 2132287785600 11 # 1 Jul 1972 2142303683200 12 # 1 Jan 1973 2152335219200 13 # 1 Jan 1974 2162366755200 14 # 1 Jan 1975 2172398291200 15 # 1 Jan 1976 2182429913600 16 # 1 Jan 1977 2192461449600 17 # 1 Jan 1978 2202492985600 18 # 1 Jan 1979 2212524521600 19 # 1 Jan 1980 2222571782400 20 # 1 Jul 1981 2232603318400 21 # 1 Jul 1982 2242634854400 22 # 1 Jul 1983 2252698012800 23 # 1 Jul 1985 2262776982400 24 # 1 Jan 1988 2272840140800 25 # 1 Jan 1990 2282871676800 26 # 1 Jan 1991 2292918937600 27 # 1 Jul 1992 2302950473600 28 # 1 Jul 1993 2312982009600 29 # 1 Jul 1994 2323029443200 30 # 1 Jan 1996 2333076704000 31 # 1 Jul 1997 2343124137600 32 # 1 Jan 1999 2353345062400 33 # 1 Jan 2006 2363439756800 34 # 1 Jan 2009 2373550089600 35 # 1 Jul 2012 2383644697600 36 # 1 Jul 2015 2393692217600 37 # 1 Jan 2017 240# 241# the following special comment contains the 242# hash value of the data in this file computed 243# use the secure hash algorithm as specified 244# by FIPS 180-1. See the files in ~/pub/sha for 245# the details of how this hash value is 246# computed. Note that the hash computation 247# ignores comments and whitespace characters 248# in data lines. It includes the NTP values 249# of both the last modification time and the 250# expiration time of the file, but not the 251# white space on those lines. 252# the hash line is also ignored in the 253# computation. 254# 255#h 599d45bf accd4b4f 8b60e46 49b623 7d13b825 256