1255902Sedwin# 2255902Sedwin# In the following text, the symbol '#' introduces 3270817Spluknet# a comment, which continues from that symbol until 4255902Sedwin# the end of the line. A plain comment line has a 5255902Sedwin# whitespace character following the comment indicator. 6270817Spluknet# There are also special comment lines defined below. 7270817Spluknet# A special comment will always have a non-whitespace 8255902Sedwin# character in column 2. 9255902Sedwin# 10255902Sedwin# A blank line should be ignored. 11255902Sedwin# 12255902Sedwin# The following table shows the corrections that must 13255902Sedwin# be applied to compute International Atomic Time (TAI) 14255902Sedwin# from the Coordinated Universal Time (UTC) values that 15255902Sedwin# are transmitted by almost all time services. 16255902Sedwin# 17255902Sedwin# The first column shows an epoch as a number of seconds 18270817Spluknet# since 1 January 1900, 00:00:00 (1900.0 is also used to 19270817Spluknet# indicate the same epoch.) Both of these time stamp formats 20270817Spluknet# ignore the complexities of the time scales that were 21270817Spluknet# used before the current definition of UTC at the start 22270817Spluknet# of 1972. (See note 3 below.) 23270817Spluknet# The second column shows the number of seconds that 24270817Spluknet# must be added to UTC to compute TAI for any timestamp 25270817Spluknet# at or after that epoch. The value on each line is 26270817Spluknet# valid from the indicated initial instant until the 27270817Spluknet# epoch given on the next one or indefinitely into the 28270817Spluknet# future if there is no next line. 29255902Sedwin# (The comment on each line shows the representation of 30270817Spluknet# the corresponding initial epoch in the usual 31255902Sedwin# day-month-year format. The epoch always begins at 32255902Sedwin# 00:00:00 UTC on the indicated day. See Note 5 below.) 33270817Spluknet# 34255902Sedwin# Important notes: 35255902Sedwin# 36255902Sedwin# 1. Coordinated Universal Time (UTC) is often referred to 37255902Sedwin# as Greenwich Mean Time (GMT). The GMT time scale is no 38255902Sedwin# longer used, and the use of GMT to designate UTC is 39255902Sedwin# discouraged. 40255902Sedwin# 41270817Spluknet# 2. The UTC time scale is realized by many national 42255902Sedwin# laboratories and timing centers. Each laboratory 43255902Sedwin# identifies its realization with its name: Thus 44255902Sedwin# UTC(NIST), UTC(USNO), etc. The differences among 45255902Sedwin# these different realizations are typically on the 46255902Sedwin# order of a few nanoseconds (i.e., 0.000 000 00x s) 47255902Sedwin# and can be ignored for many purposes. These differences 48255902Sedwin# are tabulated in Circular T, which is published monthly 49255902Sedwin# by the International Bureau of Weights and Measures 50279707Sedwin# (BIPM). See www.bipm.org for more information. 51255902Sedwin# 52270817Spluknet# 3. The current definition of the relationship between UTC 53270817Spluknet# and TAI dates from 1 January 1972. A number of different 54270817Spluknet# time scales were in use before that epoch, and it can be 55270817Spluknet# quite difficult to compute precise timestamps and time 56255902Sedwin# intervals in those "prehistoric" days. For more information, 57255902Sedwin# consult: 58255902Sedwin# 59255902Sedwin# The Explanatory Supplement to the Astronomical 60255902Sedwin# Ephemeris. 61255902Sedwin# or 62255902Sedwin# Terry Quinn, "The BIPM and the Accurate Measurement 63255902Sedwin# of Time," Proc. of the IEEE, Vol. 79, pp. 894-905, 64328476Sphilip# July, 1991. <http://dx.doi.org/10.1109/5.84965> 65328476Sphilip# reprinted in: 66328476Sphilip# Christine Hackman and Donald B Sullivan (eds.) 67328476Sphilip# Time and Frequency Measurement 68328476Sphilip# American Association of Physics Teachers (1996) 69328476Sphilip# <http://tf.nist.gov/general/pdf/1168.pdf>, pp. 75-86 70255902Sedwin# 71270817Spluknet# 4. The decision to insert a leap second into UTC is currently 72270817Spluknet# the responsibility of the International Earth Rotation and 73270817Spluknet# Reference Systems Service. (The name was changed from the 74270817Spluknet# International Earth Rotation Service, but the acronym IERS 75270817Spluknet# is still used.) 76255902Sedwin# 77270817Spluknet# Leap seconds are announced by the IERS in its Bulletin C. 78255902Sedwin# 79270817Spluknet# See www.iers.org for more details. 80255902Sedwin# 81270817Spluknet# Every national laboratory and timing center uses the 82270817Spluknet# data from the BIPM and the IERS to construct UTC(lab), 83270817Spluknet# their local realization of UTC. 84255902Sedwin# 85255902Sedwin# Although the definition also includes the possibility 86270817Spluknet# of dropping seconds ("negative" leap seconds), this has 87270817Spluknet# never been done and is unlikely to be necessary in the 88255902Sedwin# foreseeable future. 89255902Sedwin# 90255902Sedwin# 5. If your system keeps time as the number of seconds since 91255902Sedwin# some epoch (e.g., NTP timestamps), then the algorithm for 92255902Sedwin# assigning a UTC time stamp to an event that happens during a positive 93270817Spluknet# leap second is not well defined. The official name of that leap 94270817Spluknet# second is 23:59:60, but there is no way of representing that time 95270817Spluknet# in these systems. 96270817Spluknet# Many systems of this type effectively stop the system clock for 97270817Spluknet# one second during the leap second and use a time that is equivalent 98270817Spluknet# to 23:59:59 UTC twice. For these systems, the corresponding TAI 99255902Sedwin# timestamp would be obtained by advancing to the next entry in the 100255902Sedwin# following table when the time equivalent to 23:59:59 UTC 101255902Sedwin# is used for the second time. Thus the leap second which 102255902Sedwin# occurred on 30 June 1972 at 23:59:59 UTC would have TAI 103255902Sedwin# timestamps computed as follows: 104255902Sedwin# 105255902Sedwin# ... 106255902Sedwin# 30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds 107255902Sedwin# 30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds 108255902Sedwin# 1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds 109255902Sedwin# ... 110255902Sedwin# 111255902Sedwin# If your system realizes the leap second by repeating 00:00:00 UTC twice 112255902Sedwin# (this is possible but not usual), then the advance to the next entry 113270817Spluknet# in the table must occur the second time that a time equivalent to 114255902Sedwin# 00:00:00 UTC is used. Thus, using the same example as above: 115255902Sedwin# 116255902Sedwin# ... 117255902Sedwin# 30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds 118255902Sedwin# 30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds 119255902Sedwin# 1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds 120255902Sedwin# ... 121255902Sedwin# 122255902Sedwin# in both cases the use of timestamps based on TAI produces a smooth 123270817Spluknet# time scale with no discontinuity in the time interval. However, 124270817Spluknet# although the long-term behavior of the time scale is correct in both 125270817Spluknet# methods, the second method is technically not correct because it adds 126270817Spluknet# the extra second to the wrong day. 127255902Sedwin# 128270817Spluknet# This complexity would not be needed for negative leap seconds (if they 129270817Spluknet# are ever used). The UTC time would skip 23:59:59 and advance from 130270817Spluknet# 23:59:58 to 00:00:00 in that case. The TAI offset would decrease by 131270817Spluknet# 1 second at the same instant. This is a much easier situation to deal 132270817Spluknet# with, since the difficulty of unambiguously representing the epoch 133255902Sedwin# during the leap second does not arise. 134255902Sedwin# 135279707Sedwin# Some systems implement leap seconds by amortizing the leap second 136279707Sedwin# over the last few minutes of the day. The frequency of the local 137279707Sedwin# clock is decreased (or increased) to realize the positive (or 138279707Sedwin# negative) leap second. This method removes the time step described 139280414Sedwin# above. Although the long-term behavior of the time scale is correct 140280414Sedwin# in this case, this method introduces an error during the adjustment 141280414Sedwin# period both in time and in frequency with respect to the official 142280414Sedwin# definition of UTC. 143279707Sedwin# 144255902Sedwin# Questions or comments to: 145255902Sedwin# Judah Levine 146255902Sedwin# Time and Frequency Division 147255902Sedwin# NIST 148255902Sedwin# Boulder, Colorado 149270817Spluknet# Judah.Levine@nist.gov 150255902Sedwin# 151307362Sbapt# Last Update of leap second values: 8 July 2016 152255902Sedwin# 153270817Spluknet# The following line shows this last update date in NTP timestamp 154255902Sedwin# format. This is the date on which the most recent change to 155255902Sedwin# the leap second data was added to the file. This line can 156270817Spluknet# be identified by the unique pair of characters in the first two 157255902Sedwin# columns as shown below. 158255902Sedwin# 159307362Sbapt#$ 3676924800 160255902Sedwin# 161255902Sedwin# The NTP timestamps are in units of seconds since the NTP epoch, 162270817Spluknet# which is 1 January 1900, 00:00:00. The Modified Julian Day number 163270817Spluknet# corresponding to the NTP time stamp, X, can be computed as 164255902Sedwin# 165255902Sedwin# X/86400 + 15020 166255902Sedwin# 167270817Spluknet# where the first term converts seconds to days and the second 168270817Spluknet# term adds the MJD corresponding to the time origin defined above. 169270817Spluknet# The integer portion of the result is the integer MJD for that 170270817Spluknet# day, and any remainder is the time of day, expressed as the 171270817Spluknet# fraction of the day since 0 hours UTC. The conversion from day 172270817Spluknet# fraction to seconds or to hours, minutes, and seconds may involve 173270817Spluknet# rounding or truncation, depending on the method used in the 174270817Spluknet# computation. 175255902Sedwin# 176270817Spluknet# The data in this file will be updated periodically as new leap 177255902Sedwin# seconds are announced. In addition to being entered on the line 178270817Spluknet# above, the update time (in NTP format) will be added to the basic 179255902Sedwin# file name leap-seconds to form the name leap-seconds.<NTP TIME>. 180270817Spluknet# In addition, the generic name leap-seconds.list will always point to 181255902Sedwin# the most recent version of the file. 182255902Sedwin# 183255902Sedwin# This update procedure will be performed only when a new leap second 184270817Spluknet# is announced. 185255902Sedwin# 186255902Sedwin# The following entry specifies the expiration date of the data 187270817Spluknet# in this file in units of seconds since the origin at the instant 188270817Spluknet# 1 January 1900, 00:00:00. This expiration date will be changed 189270817Spluknet# at least twice per year whether or not a new leap second is 190270817Spluknet# announced. These semi-annual changes will be made no later 191270817Spluknet# than 1 June and 1 December of each year to indicate what 192270817Spluknet# action (if any) is to be taken on 30 June and 31 December, 193255902Sedwin# respectively. (These are the customary effective dates for new 194255902Sedwin# leap seconds.) This expiration date will be identified by a 195255902Sedwin# unique pair of characters in columns 1 and 2 as shown below. 196270817Spluknet# In the unlikely event that a leap second is announced with an 197255902Sedwin# effective date other than 30 June or 31 December, then this 198255902Sedwin# file will be edited to include that leap second as soon as it is 199255902Sedwin# announced or at least one month before the effective date 200270817Spluknet# (whichever is later). 201270817Spluknet# If an announcement by the IERS specifies that no leap second is 202270817Spluknet# scheduled, then only the expiration date of the file will 203255902Sedwin# be advanced to show that the information in the file is still 204270817Spluknet# current -- the update time stamp, the data and the name of the file 205255902Sedwin# will not change. 206255902Sedwin# 207345671Sphilip# Updated through IERS Bulletin C57 208345671Sphilip# File expires on: 28 December 2019 209255902Sedwin# 210345671Sphilip#@ 3786480000 211255902Sedwin# 212255902Sedwin2272060800 10 # 1 Jan 1972 213255902Sedwin2287785600 11 # 1 Jul 1972 214255902Sedwin2303683200 12 # 1 Jan 1973 215255902Sedwin2335219200 13 # 1 Jan 1974 216255902Sedwin2366755200 14 # 1 Jan 1975 217255902Sedwin2398291200 15 # 1 Jan 1976 218255902Sedwin2429913600 16 # 1 Jan 1977 219255902Sedwin2461449600 17 # 1 Jan 1978 220255902Sedwin2492985600 18 # 1 Jan 1979 221255902Sedwin2524521600 19 # 1 Jan 1980 222255902Sedwin2571782400 20 # 1 Jul 1981 223255902Sedwin2603318400 21 # 1 Jul 1982 224255902Sedwin2634854400 22 # 1 Jul 1983 225255902Sedwin2698012800 23 # 1 Jul 1985 226255902Sedwin2776982400 24 # 1 Jan 1988 227255902Sedwin2840140800 25 # 1 Jan 1990 228255902Sedwin2871676800 26 # 1 Jan 1991 229255902Sedwin2918937600 27 # 1 Jul 1992 230255902Sedwin2950473600 28 # 1 Jul 1993 231255902Sedwin2982009600 29 # 1 Jul 1994 232255902Sedwin3029443200 30 # 1 Jan 1996 233255902Sedwin3076704000 31 # 1 Jul 1997 234255902Sedwin3124137600 32 # 1 Jan 1999 235255902Sedwin3345062400 33 # 1 Jan 2006 236255902Sedwin3439756800 34 # 1 Jan 2009 237255902Sedwin3550089600 35 # 1 Jul 2012 238279707Sedwin3644697600 36 # 1 Jul 2015 239307362Sbapt3692217600 37 # 1 Jan 2017 240255902Sedwin# 241255902Sedwin# the following special comment contains the 242255902Sedwin# hash value of the data in this file computed 243255902Sedwin# use the secure hash algorithm as specified 244255902Sedwin# by FIPS 180-1. See the files in ~/pub/sha for 245255902Sedwin# the details of how this hash value is 246255902Sedwin# computed. Note that the hash computation 247255902Sedwin# ignores comments and whitespace characters 248255902Sedwin# in data lines. It includes the NTP values 249270817Spluknet# of both the last modification time and the 250255902Sedwin# expiration time of the file, but not the 251255902Sedwin# white space on those lines. 252255902Sedwin# the hash line is also ignored in the 253255902Sedwin# computation. 254255902Sedwin# 255345671Sphilip#h 83c68138 d3650221 07dbbbcd 11fcc859 ced1106a 256