# ATOMIC TIME # 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 led # 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's rotation since the latter # undergoes a full range of geophysical perturbations at various time scales: lunisolar # and core-mantle torques, atmospheric and oceanic effects, etc. # Leap seconds are needed to keep the two time scales in agreement, i.e. UT1-UTC smaller # than 0.9 seconds. Therefore, when necessary a "leap second" is applied to UTC. # Since the adoption of this system in 1972 it has been necessary to add a number of 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 # theoretically 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 the year 2000, it was necessary in average to add a # leap second at a rate of 1 to 2 years. Since the year 2000 leap seconds are introduced with an # average interval of 3 to 4 years due to the acceleration of the Earth's rotation speed. # # # RESPONSIBILITY 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 seconds should be scheduled only for certain dates: # first preference is given to the end of December and June, and second preference at the end of March # and September. Since the introduction of leap seconds in 1972, only dates in June and December were used. # # Questions or comments to: # Christian Bizouard: christian.bizouard@obspm.fr # Earth orientation Center of the IERS # Paris Observatory, France # # # # COPYRIGHT STATUS OF THIS FILE # This file is in the public domain. # # # 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 (https://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat) # # The following line shows the last update of this file in NTP timestamp: # #$ 3945196800 # # 2) Expiration date of the file given on a semi-annual basis: last June or last December # # File expires on 28 December 2025 # # Expire date in NTP timestamp: # #@ 3975868800 # # # 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 3692217600 37 # 1 Jan 2017 # # A hash code has been generated to be able to verify the integrity # of this file. For more information about using this hash code, # please see the readme file in the 'source' directory : # https://hpiers.obspm.fr/iers/bul/bulc/ntp/sources/README # #h 848434d5 570f7ea8 d79ba227 a00fc821 f608e2d4

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