leap-seconds revision 296373
1153816Sdougb# 2234010Sdougb# $FreeBSD: releng/10.3/etc/ntp/leap-seconds 295461 2016-02-10 07:16:17Z cy $ 3153816Sdougb# 4153816Sdougb# In the following text, the symbol '#' introduces 5204619Sdougb# a comment, which continues from that symbol until 6153816Sdougb# the end of the line. A plain comment line has a 7153816Sdougb# whitespace character following the comment indicator. 8153816Sdougb# There are also special comment lines defined below. 9153816Sdougb# A special comment will always have a non-whitespace 10153816Sdougb# character in column 2. 11153816Sdougb# 12153816Sdougb# A blank line should be ignored. 13153816Sdougb# 14153816Sdougb# The following table shows the corrections that must 15153816Sdougb# be applied to compute International Atomic Time (TAI) 16153816Sdougb# from the Coordinated Universal Time (UTC) values that 17234010Sdougb# are transmitted by almost all time services. 18153816Sdougb# 19153816Sdougb# The first column shows an epoch as a number of seconds 20153816Sdougb# since 1900.0 and the second column shows the number of 21153816Sdougb# seconds that must be added to UTC to compute TAI for 22170222Sdougb# any timestamp at or after that epoch. The value on 23153816Sdougb# each line is valid from the indicated initial instant 24170222Sdougb# until the epoch given on the next one or indefinitely 25153816Sdougb# into the future if there is no next line. 26153816Sdougb# (The comment on each line shows the representation of 27153816Sdougb# the corresponding initial epoch in the usual 28153816Sdougb# day-month-year format. The epoch always begins at 29153816Sdougb# 00:00:00 UTC on the indicated day. See Note 5 below.) 30153816Sdougb# 31153816Sdougb# Important notes: 32153816Sdougb# 33153816Sdougb# 1. Coordinated Universal Time (UTC) is often referred to 34153816Sdougb# as Greenwich Mean Time (GMT). The GMT time scale is no 35153816Sdougb# longer used, and the use of GMT to designate UTC is 36153816Sdougb# discouraged. 37153816Sdougb# 38153816Sdougb# 2. The UTC time scale is realized by many national 39153816Sdougb# laboratories and timing centers. Each laboratory 40153816Sdougb# identifies its realization with its name: Thus 41153816Sdougb# UTC(NIST), UTC(USNO), etc. The differences among 42153816Sdougb# these different realizations are typically on the 43234010Sdougb# order of a few nanoseconds (i.e., 0.000 000 00x s) 44234010Sdougb# and can be ignored for many purposes. These differences 45153816Sdougb# are tabulated in Circular T, which is published monthly 46153816Sdougb# by the International Bureau of Weights and Measures 47153816Sdougb# (BIPM). See www.bipm.fr for more information. 48153816Sdougb# 49153816Sdougb# 3. The current defintion of the relationship between UTC 50153816Sdougb# and TAI dates from 1 January 1972. A number of different 51153816Sdougb# time scales were in use before than epoch, and it can be 52170222Sdougb# quite difficult to compute precise timestamps and time 53153816Sdougb# intervals in those "prehistoric" days. For more information, 54234010Sdougb# consult: 55234010Sdougb# 56234010Sdougb# The Explanatory Supplement to the Astronomical 57234010Sdougb# Ephemeris. 58153816Sdougb# or 59234010Sdougb# Terry Quinn, "The BIPM and the Accurate Measurement 60234010Sdougb# of Time," Proc. of the IEEE, Vol. 79, pp. 894-905, 61234010Sdougb# July, 1991. 62234010Sdougb# 63234010Sdougb# 4. The insertion of leap seconds into UTC is currently the 64234010Sdougb# responsibility of the International Earth Rotation Service, 65153816Sdougb# which is located at the Paris Observatory: 66153816Sdougb# 67165071Sdougb# Central Bureau of IERS 68153816Sdougb# 61, Avenue de l'Observatoire 69234010Sdougb# 75014 Paris, France. 70234010Sdougb# 71234010Sdougb# Leap seconds are announced by the IERS in its Bulletin C 72234010Sdougb# 73234010Sdougb# See hpiers.obspm.fr or www.iers.org for more details. 74153816Sdougb# 75153816Sdougb# All national laboratories and timing centers use the 76153816Sdougb# data from the BIPM and the IERS to construct their 77153816Sdougb# local realizations of UTC. 78153816Sdougb# 79234010Sdougb# Although the definition also includes the possibility 80234010Sdougb# of dropping seconds ("negative" leap seconds), this has 81153816Sdougb# never been done and is unlikely to be necessary in the 82234010Sdougb# foreseeable future. 83234010Sdougb# 84153816Sdougb# 5. If your system keeps time as the number of seconds since 85234010Sdougb# some epoch (e.g., NTP timestamps), then the algorithm for 86234010Sdougb# assigning a UTC time stamp to an event that happens during a positive 87153816Sdougb# leap second is not well defined. The official name of that leap 88153816Sdougb# second is 23:59:60, but there is no way of representing that time 89153816Sdougb# in these systems. 90153816Sdougb# Many systems of this type effectively stop the system clock for 91153816Sdougb# one second during the leap second and use a time that is equivalent 92153816Sdougb# to 23:59:59 UTC twice. For these systems, the corresponding TAI 93153816Sdougb# timestamp would be obtained by advancing to the next entry in the 94153816Sdougb# following table when the time equivalent to 23:59:59 UTC 95234010Sdougb# is used for the second time. Thus the leap second which 96234010Sdougb# occurred on 30 June 1972 at 23:59:59 UTC would have TAI 97153816Sdougb# timestamps computed as follows: 98153816Sdougb# 99234010Sdougb# ... 100234010Sdougb# 30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds 101234010Sdougb# 30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds 102234010Sdougb# 1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds 103234010Sdougb# ... 104234010Sdougb# 105153816Sdougb# If your system realizes the leap second by repeating 00:00:00 UTC twice 106234010Sdougb# (this is possible but not usual), then the advance to the next entry 107234010Sdougb# in the table must occur the second time that a time equivlent to 108153816Sdougb# 00:00:00 UTC is used. Thus, using the same example as above: 109153816Sdougb# 110234010Sdougb# ... 111234010Sdougb# 30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds 112234010Sdougb# 30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds 113153816Sdougb# 1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds 114224092Sdougb# ... 115153816Sdougb# 116234010Sdougb# in both cases the use of timestamps based on TAI produces a smooth 117234010Sdougb# time scale with no discontinuity in the time interval. 118234010Sdougb# 119234010Sdougb# This complexity would not be needed for negative leap seconds (if they 120234010Sdougb# are ever used). The UTC time would skip 23:59:59 and advance from 121234010Sdougb# 23:59:58 to 00:00:00 in that case. The TAI offset would decrease by 122234010Sdougb# 1 second at the same instant. This is a much easier situation to deal 123234010Sdougb# with, since the difficulty of unambiguously representing the epoch 124234010Sdougb# during the leap second does not arise. 125234010Sdougb# 126234010Sdougb# Questions or comments to: 127234010Sdougb# Jeff Prillaman 128234010Sdougb# Time Service Department 129153816Sdougb# US Naval Observatory 130224092Sdougb# Washington, DC 131224092Sdougb# jeffrey.prillaman@usno.navy.mil 132234010Sdougb# 133234010Sdougb# Last Update of leap second values: 31 Dec 2015 134153816Sdougb# 135224092Sdougb# The following line shows this last update date in NTP timestamp 136224092Sdougb# format. This is the date on which the most recent change to 137234010Sdougb# the leap second data was added to the file. This line can 138234010Sdougb# be identified by the unique pair of characters in the first two 139234010Sdougb# columns as shown below. 140234010Sdougb# 141234010Sdougb#$ 3660508800 142234010Sdougb# 143224092Sdougb# The data in this file will be updated periodically as new leap 144234010Sdougb# seconds are announced. In addition to being entered on the line 145224092Sdougb# above, the update time (in NTP format) will be added to the basic 146234010Sdougb# file name leap-seconds to form the name leap-seconds.<NTP TIME>. 147234010Sdougb# In addition, the generic name leap-seconds.list will always point to 148224092Sdougb# the most recent version of the file. 149224092Sdougb# 150165071Sdougb# This update procedure will be performed only when a new leap second 151153816Sdougb# is announced. 152234010Sdougb# 153153816Sdougb# The following entry specifies the expiration date of the data 154153816Sdougb# in this file in units of seconds since 1900.0. This expiration date 155165071Sdougb# will be changed at least twice per year whether or not a new leap 156153816Sdougb# second is announced. These semi-annual changes will be made no 157153816Sdougb# later than 1 June and 1 December of each year to indicate what 158153816Sdougb# action (if any) is to be taken on 30 June and 31 December, 159153816Sdougb# respectively. (These are the customary effective dates for new 160234010Sdougb# leap seconds.) This expiration date will be identified by a 161153816Sdougb# unique pair of characters in columns 1 and 2 as shown below. 162153816Sdougb# In the unlikely event that a leap second is announced with an 163153816Sdougb# effective date other than 30 June or 31 December, then this 164234010Sdougb# file will be edited to include that leap second as soon as it is 165153816Sdougb# announced or at least one month before the effective date 166170222Sdougb# (whichever is later). 167234010Sdougb# If an announcement by the IERS specifies that no leap second is 168170222Sdougb# scheduled, then only the expiration date of the file will 169170222Sdougb# be advanced to show that the information in the file is still 170234010Sdougb# current -- the update time stamp, the data and the name of the file 171234010Sdougb# will not change. 172170222Sdougb# 173234010Sdougb# Updated through IERS Bulletin C 50 174234010Sdougb# File expires on: 1 Jun 2016 175234010Sdougb# 176234010Sdougb#@ 3673728000 177170222Sdougb# 178234010Sdougb2272060800 10 # 1 Jan 1972 179234010Sdougb2287785600 11 # 1 Jul 1972 180234010Sdougb2303683200 12 # 1 Jan 1973 181234010Sdougb2335219200 13 # 1 Jan 1974 182170222Sdougb2366755200 14 # 1 Jan 1975 183234010Sdougb2398291200 15 # 1 Jan 1976 184170222Sdougb2429913600 16 # 1 Jan 1977 185170222Sdougb2461449600 17 # 1 Jan 1978 186153816Sdougb2492985600 18 # 1 Jan 1979 187170222Sdougb2524521600 19 # 1 Jan 1980 188170222Sdougb2571782400 20 # 1 Jul 1981 189193149Sdougb2603318400 21 # 1 Jul 1982 190234010Sdougb2634854400 22 # 1 Jul 1983 191193149Sdougb2698012800 23 # 1 Jul 1985 192224092Sdougb2776982400 24 # 1 Jan 1988 193234010Sdougb2840140800 25 # 1 Jan 1990 194170222Sdougb2871676800 26 # 1 Jan 1991 195234010Sdougb2918937600 27 # 1 Jul 1992 196224092Sdougb2950473600 28 # 1 Jul 1993 197224092Sdougb2982009600 29 # 1 Jul 1994 198153816Sdougb3029443200 30 # 1 Jan 1996 199234010Sdougb3076704000 31 # 1 Jul 1997 200153816Sdougb3124137600 32 # 1 Jan 1999 201170222Sdougb3345062400 33 # 1 Jan 2006 202234010Sdougb3439756800 34 # 1 Jan 2009 203153816Sdougb3550089600 35 # 1 Jul 2012 204234010Sdougb3644697600 36 # 1 Jul 2015 205153816Sdougb# 206153816Sdougb# the following special comment contains the 207234010Sdougb# hash value of the data in this file computed 208153816Sdougb# use the secure hash algorithm as specified 209234010Sdougb# by FIPS 180-1. See the files in ~/sha for 210234010Sdougb# the details of how this hash value is 211234010Sdougb# computed. Note that the hash computation 212170222Sdougb# ignores comments and whitespace characters 213153816Sdougb# in data lines. It includes the NTP values 214193149Sdougb# of both the last modification time and the 215193149Sdougb# expiration time of the file, but not the 216153816Sdougb# white space on those lines. 217165071Sdougb# the hash line is also ignored in the 218153816Sdougb# computation. 219153816Sdougb# 220234010Sdougb#h 44a44c49 35b22601 a9c7054c 8c56cf57 9b6f6ed5 221153816Sdougb# 222234010Sdougb