leap-seconds revision 289421 
1#
2# $FreeBSD: head/etc/ntp/leap-seconds 289421 2015-10-16 14:04:16Z cy $
3#
4#	ATOMIC TIME.
5#	The Coordinated Universal Time (UTC) is the reference time scale derived 
6#	from The "Temps Atomique International" (TAI) calculated by the Bureau 
7#	International des Poids et Mesures (BIPM) using a worldwide network of atomic 
8#	clocks. UTC differs from TAI by an integer number of seconds; it is the basis 
9#	of all activities in the world. 
10#
11#
12#	ASTRONOMICAL TIME (UT1) is the time scale based on the rate of rotation of the earth. 
13#	It is now mainly derived from Very Long Baseline Interferometry (VLBI). The various 
14#	irregular fluctuations progressively detected in the rotation rate of the Earth lead 
15#	in 1972 to the replacement of UT1 by UTC as the reference time scale. 
16#
17#
18#	LEAP SECOND
19#	Atomic clocks are more stable than the rate of the earth rotatiob since the later 
20#	undergoes a full range of geophysical perturbations at various time scales (lunisolar 
21#	and core-mantle torques,atmospheric and oceanic effetcs, ...)
22#	Leap seconds are needed to keep the two time scales in agreement, i.e. UT1-UTC smaller 
23#	than 0.9 second. So, when necessary a "leap second" is introduced in UTC.
24#	Since the adoption of this system in 1972 it has been necessary to add 26 seconds to UTC, 
25#	firstly due to the initial choice of the value of the second (1/86400 mean solar day of 
26#	the year 1820) and secondly to the general slowing down of the Earth's rotation. It is 
27#	theorically possible to have a negative leap second (a second removed from UTC), but so far, 
28#	all leap seconds have been positive (a second has been added to UTC). Based on what we know about the earth's rotation, 
29#	it is unlikely that we will ever have a negative leap second.
30#
31#
32#	HISTORY
33#	The first leap second was added on June 30, 1972. Until 2000, it was necessary in average to add a leap second at a rate 
34#	of 1 to 2 years. Since 2000, due to the fact that the earth rate of rotation is accelerating, leap seconds are introduced 
35#	with an average  frequency of 3 to 4 years.
36#
37#
38#	RESPONSABILITY OF THE DECISION TO INTRODUCE A LEAP SECOND IN UTC
39#	The decision to introduce a leap second in UTC is the responsibility of the Earth Orientation Center of 
40#	the International Earth Rotation and reference System Service (IERS). This center is located at Paris 
41#	Observatory. According to international agreements, leap second date have to occur at fixed date :
42#	first preference is given to the end of December and June, and second preference at the end of March 
43#	and September. Since the system was introduced in 1972, only dates in June and December were used.
44#
45#		Questions or comments to:
46#			Daniel Gambis, daniel.gambis@obspm.fr
47#			Christian Bizouard:  christian.bizouard@obspm.fr
48#			Earth orientation Center of the IERS
49#			Paris Observatory, France	
50#			
51#
52#
53#	VALIDITY OF THE FILE
54#	It is important to express the validity of the file. These next two dates are
55#	given in units of seconds since 1900.0.
56#
57#	1) Last update of the file. 
58#
59#	Updated through IERS Bulletin C (ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat)
60#
61#	The following line shows the last update of this file in NTP timestamp: 
62#
63#$	3645216000 
64#	 
65#	2) Expiration date of the file given on a semi-annual basis: last June or last December
66#
67#	File expires on 28 December 2015
68#
69#	Expire date in NTP timestamp: 
70#
71#@	3660249600
72#
73#
74#	LIST OF LEAP SECONDS
75#	NTP timestamp (X parameter) is the number of seconds since 1900.0
76#
77#	MJD: The Modified Julian Day number. MJD = X/86400 + 15020
78#
79#	DTAI: The difference DTAI= TAI-UTC in units of seconds
80#	It is the quantity to add to UTC to get the time in TAI
81#
82#	Day Month Year : epoch in clear
83#
84#NTP Time      DTAI    Day Month Year
85#
862272060800      10      # 1 Jan 1972
872287785600      11      # 1 Jul 1972
882303683200      12      # 1 Jan 1973
892335219200      13      # 1 Jan 1974
902366755200      14      # 1 Jan 1975
912398291200      15      # 1 Jan 1976
922429913600      16      # 1 Jan 1977
932461449600      17      # 1 Jan 1978
942492985600      18      # 1 Jan 1979
952524521600      19      # 1 Jan 1980
962571782400      20      # 1 Jul 1981
972603318400      21      # 1 Jul 1982
982634854400      22      # 1 Jul 1983
992698012800      23      # 1 Jul 1985
1002776982400      24      # 1 Jan 1988
1012840140800      25      # 1 Jan 1990
1022871676800      26      # 1 Jan 1991
1032918937600      27      # 1 Jul 1992
1042950473600      28      # 1 Jul 1993
1052982009600      29      # 1 Jul 1994
1063029443200      30      # 1 Jan 1996
1073076704000      31      # 1 Jul 1997
1083124137600      32      # 1 Jan 1999
1093345062400      33      # 1 Jan 2006
1103439756800      34      # 1 Jan 2009
1113550089600      35      # 1 Jul 2012
1123644697600      36      # 1 Jul 2015
113#
114#	In order to verify the integrity of this file, a hash code 
115#	has been generated. For more information how to use
116#	this hash code, please consult the README file under the 
117#	'sha' repertory.
118#
119#h	620ba8af 37900668 95ac09ba d77640f9 6fd75493
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