xref: /freebsd-11.0-release/etc/ntp/leap-seconds

#
# $FreeBSD: head/etc/ntp/leap-seconds 289421 2015-10-16 14:04:16Z cy $
#
#	ATOMIC TIME.
#	The Coordinated Universal Time (UTC) is the reference time scale derived 
#	from The "Temps Atomique International" (TAI) calculated by the Bureau 
#	International des Poids et Mesures (BIPM) using a worldwide network of atomic 
#	clocks. UTC differs from TAI by an integer number of seconds; it is the basis 
#	of all activities in the world. 
#
#
#	ASTRONOMICAL TIME (UT1) is the time scale based on the rate of rotation of the earth. 
#	It is now mainly derived from Very Long Baseline Interferometry (VLBI). The various 
#	irregular fluctuations progressively detected in the rotation rate of the Earth lead 
#	in 1972 to the replacement of UT1 by UTC as the reference time scale. 
#
#
#	LEAP SECOND
#	Atomic clocks are more stable than the rate of the earth rotatiob since the later 
#	undergoes a full range of geophysical perturbations at various time scales (lunisolar 
#	and core-mantle torques,atmospheric and oceanic effetcs, ...)
#	Leap seconds are needed to keep the two time scales in agreement, i.e. UT1-UTC smaller 
#	than 0.9 second. So, when necessary a "leap second" is introduced in UTC.
#	Since the adoption of this system in 1972 it has been necessary to add 26 seconds to UTC, 
#	firstly due to the initial choice of the value of the second (1/86400 mean solar day of 
#	the year 1820) and secondly to the general slowing down of the Earth's rotation. It is 
#	theorically possible to have a negative leap second (a second removed from UTC), but so far, 
#	all leap seconds have been positive (a second has been added to UTC). Based on what we know about the earth's rotation, 
#	it is unlikely that we will ever have a negative leap second.
#
#
#	HISTORY
#	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 
#	of 1 to 2 years. Since 2000, due to the fact that the earth rate of rotation is accelerating, leap seconds are introduced 
#	with an average  frequency of 3 to 4 years.
#
#
#	RESPONSABILITY OF THE DECISION TO INTRODUCE A LEAP SECOND IN UTC
#	The decision to introduce a leap second in UTC is the responsibility of the Earth Orientation Center of 
#	the International Earth Rotation and reference System Service (IERS). This center is located at Paris 
#	Observatory. According to international agreements, leap second date have to occur at fixed date :
#	first preference is given to the end of December and June, and second preference at the end of March 
#	and September. Since the system was introduced in 1972, only dates in June and December were used.
#
#		Questions or comments to:
#			Daniel Gambis, daniel.gambis@obspm.fr
#			Christian Bizouard:  christian.bizouard@obspm.fr
#			Earth orientation Center of the IERS
#			Paris Observatory, France	
#			
#
#
#	VALIDITY OF THE FILE
#	It is important to express the validity of the file. These next two dates are
#	given in units of seconds since 1900.0.
#
#	1) Last update of the file. 
#
#	Updated through IERS Bulletin C (ftp://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat)
#
#	The following line shows the last update of this file in NTP timestamp: 
#
#$	3645216000 
#	 
#	2) Expiration date of the file given on a semi-annual basis: last June or last December
#
#	File expires on 28 December 2015
#
#	Expire date in NTP timestamp: 
#
#@	3660249600
#
#
#	LIST OF LEAP SECONDS
#	NTP timestamp (X parameter) is the number of seconds since 1900.0
#
#	MJD: The Modified Julian Day number. MJD = X/86400 + 15020
#
#	DTAI: The difference DTAI= TAI-UTC in units of seconds
#	It is the quantity to add to UTC to get the time in TAI
#
#	Day Month Year : epoch in clear
#
#NTP Time      DTAI    Day Month Year
#
2272060800      10      # 1 Jan 1972
2287785600      11      # 1 Jul 1972
2303683200      12      # 1 Jan 1973
2335219200      13      # 1 Jan 1974
2366755200      14      # 1 Jan 1975
2398291200      15      # 1 Jan 1976
2429913600      16      # 1 Jan 1977
2461449600      17      # 1 Jan 1978
2492985600      18      # 1 Jan 1979
2524521600      19      # 1 Jan 1980
2571782400      20      # 1 Jul 1981
2603318400      21      # 1 Jul 1982
2634854400      22      # 1 Jul 1983
2698012800      23      # 1 Jul 1985
2776982400      24      # 1 Jan 1988
2840140800      25      # 1 Jan 1990
2871676800      26      # 1 Jan 1991
2918937600      27      # 1 Jul 1992
2950473600      28      # 1 Jul 1993
2982009600      29      # 1 Jul 1994
3029443200      30      # 1 Jan 1996
3076704000      31      # 1 Jul 1997
3124137600      32      # 1 Jan 1999
3345062400      33      # 1 Jan 2006
3439756800      34      # 1 Jan 2009
3550089600      35      # 1 Jul 2012
3644697600      36      # 1 Jul 2015
#
#	In order to verify the integrity of this file, a hash code 
#	has been generated. For more information how to use
#	this hash code, please consult the README file under the 
#	'sha' repertory.
#
#h	620ba8af 37900668 95ac09ba d77640f9 6fd75493