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Editing: leap-seconds

#
#	In the following text, the symbol '#' introduces
#	a comment, which continues from that symbol until
#	the end of the line. A plain comment line has a
#	whitespace character following the comment indicator.
#	There are also special comment lines defined below.
#	A special comment will always have a non-whitespace
#	character in column 2.
#
#	A blank line should be ignored.
#
#	The following table shows the corrections that must
#	be applied to compute International Atomic Time (TAI)
#	from the Coordinated Universal Time (UTC) values that
#	are transmitted by almost all time services.
#
#	The first column shows an epoch as a number of seconds
#	since 1 January 1900, 00:00:00 (1900.0 is also used to
#	indicate the same epoch.) Both of these time stamp formats
#	ignore the complexities of the time scales that were
#	used before the current definition of UTC at the start
#	of 1972. (See note 3 below.)
#	The second column shows the number of seconds that
#	must be added to UTC to compute TAI for any timestamp
#	at or after that epoch. The value on each line is
#	valid from the indicated initial instant until the
#	epoch given on the next one or indefinitely into the
#	future if there is no next line.
#	(The comment on each line shows the representation of
#	the corresponding initial epoch in the usual
#	day-month-year format. The epoch always begins at
#	00:00:00 UTC on the indicated day. See Note 5 below.)
#
#	Important notes:
#
#	1. Coordinated Universal Time (UTC) is often referred to
#	as Greenwich Mean Time (GMT). The GMT time scale is no
#	longer used, and the use of GMT to designate UTC is
#	discouraged.
#
#	2. The UTC time scale is realized by many national
#	laboratories and timing centers. Each laboratory
#	identifies its realization with its name: Thus
#	UTC(NIST), UTC(USNO), etc. The differences among
#	these different realizations are typically on the
#	order of a few nanoseconds (i.e., 0.000 000 00x s)
#	and can be ignored for many purposes. These differences
#	are tabulated in Circular T, which is published monthly
#	by the International Bureau of Weights and Measures
#	(BIPM). See www.bipm.org for more information.
#
#	3. The current definition of the relationship between UTC
#	and TAI dates from 1 January 1972. A number of different
#	time scales were in use before that epoch, and it can be
#	quite difficult to compute precise timestamps and time
#	intervals in those "prehistoric" days. For more information,
#	consult:
#
#		The Explanatory Supplement to the Astronomical
#		Ephemeris.
#	or
#		Terry Quinn, "The BIPM and the Accurate Measurement
#		of Time," Proc. of the IEEE, Vol. 79, pp. 894-905,
#		July, 1991. <http://dx.doi.org/10.1109/5.84965>
#		reprinted in:
#		   Christine Hackman and Donald B Sullivan (eds.)
#		   Time and Frequency Measurement
#		   American Association of Physics Teachers (1996)
#		   <http://tf.nist.gov/general/pdf/1168.pdf>, pp. 75-86
#
#	4. The decision to insert a leap second into UTC is currently
#	the responsibility of the International Earth Rotation and
#	Reference Systems Service. (The name was changed from the
#	International Earth Rotation Service, but the acronym IERS
#	is still used.)
#
#	Leap seconds are announced by the IERS in its Bulletin C.
#
#	See www.iers.org for more details.
#
#	Every national laboratory and timing center uses the
#	data from the BIPM and the IERS to construct UTC(lab),
#	their local realization of UTC.
#
#	Although the definition also includes the possibility
#	of dropping seconds ("negative" leap seconds), this has
#	never been done and is unlikely to be necessary in the
#	foreseeable future.
#
#	5. If your system keeps time as the number of seconds since
#	some epoch (e.g., NTP timestamps), then the algorithm for
#	assigning a UTC time stamp to an event that happens during a positive
#	leap second is not well defined. The official name of that leap
#	second is 23:59:60, but there is no way of representing that time
#	in these systems.
#	Many systems of this type effectively stop the system clock for
#	one second during the leap second and use a time that is equivalent
#	to 23:59:59 UTC twice. For these systems, the corresponding TAI
#	timestamp would be obtained by advancing to the next entry in the
#	following table when the time equivalent to 23:59:59 UTC
#	is used for the second time. Thus the leap second which
#	occurred on 30 June 1972 at 23:59:59 UTC would have TAI
#	timestamps computed as follows:
#
#	...
#	30 June 1972 23:59:59 (2287785599, first time):	TAI= UTC + 10 seconds
#	30 June 1972 23:59:60 (2287785599,second time):	TAI= UTC + 11 seconds
#	1  July 1972 00:00:00 (2287785600)		TAI= UTC + 11 seconds
#	...
#
#	If your system realizes the leap second by repeating 00:00:00 UTC twice
#	(this is possible but not usual), then the advance to the next entry
#	in the table must occur the second time that a time equivalent to
#	00:00:00 UTC is used. Thus, using the same example as above:
#
#	...
#       30 June 1972 23:59:59 (2287785599):		TAI= UTC + 10 seconds
#       30 June 1972 23:59:60 (2287785600, first time):	TAI= UTC + 10 seconds
#       1  July 1972 00:00:00 (2287785600,second time):	TAI= UTC + 11 seconds
#	...
#
#	in both cases the use of timestamps based on TAI produces a smooth
#	time scale with no discontinuity in the time interval. However,
#	although the long-term behavior of the time scale is correct in both
#	methods, the second method is technically not correct because it adds
#	the extra second to the wrong day.
#
#	This complexity would not be needed for negative leap seconds (if they
#	are ever used). The UTC time would skip 23:59:59 and advance from
#	23:59:58 to 00:00:00 in that case. The TAI offset would decrease by
#	1 second at the same instant. This is a much easier situation to deal
#	with, since the difficulty of unambiguously representing the epoch
#	during the leap second does not arise.
#
#	Some systems implement leap seconds by amortizing the leap second
#	over the last few minutes of the day. The frequency of the local
#	clock is decreased (or increased) to realize the positive (or
#	negative) leap second. This method removes the time step described
#	above. Although the long-term behavior of the time scale is correct
#	in this case, this method introduces an error during the adjustment
#	period both in time and in frequency with respect to the official
#	definition of UTC.
#
#	Questions or comments to:
#		Judah Levine
#		Time and Frequency Division
#		NIST
#		Boulder, Colorado
#		Judah.Levine@nist.gov
#
#	Last Update of leap second values:   8 July 2016
#
#	The following line shows this last update date in NTP timestamp
#	format. This is the date on which the most recent change to
#	the leap second data was added to the file. This line can
#	be identified by the unique pair of characters in the first two
#	columns as shown below.
#
#$	 3676924800
#
#	The NTP timestamps are in units of seconds since the NTP epoch,
#	which is 1 January 1900, 00:00:00. The Modified Julian Day number
#	corresponding to the NTP time stamp, X, can be computed as
#
#	X/86400 + 15020
#
#	where the first term converts seconds to days and the second
#	term adds the MJD corresponding to the time origin defined above.
#	The integer portion of the result is the integer MJD for that
#	day, and any remainder is the time of day, expressed as the
#	fraction of the day since 0 hours UTC. The conversion from day
#	fraction to seconds or to hours, minutes, and seconds may involve
#	rounding or truncation, depending on the method used in the
#	computation.
#
#	The data in this file will be updated periodically as new leap
#	seconds are announced. In addition to being entered on the line
#	above, the update time (in NTP format) will be added to the basic
#	file name leap-seconds to form the name leap-seconds.<NTP TIME>.
#	In addition, the generic name leap-seconds.list will always point to
#	the most recent version of the file.
#
#	This update procedure will be performed only when a new leap second
#	is announced.
#
#	The following entry specifies the expiration date of the data
#	in this file in units of seconds since the origin at the instant
#	1 January 1900, 00:00:00. This expiration date will be changed
#	at least twice per year whether or not a new leap second is
#	announced. These semi-annual changes will be made no later
#	than 1 June and 1 December of each year to indicate what
#	action (if any) is to be taken on 30 June and 31 December,
#	respectively. (These are the customary effective dates for new
#	leap seconds.) This expiration date will be identified by a
#	unique pair of characters in columns 1 and 2 as shown below.
#	In the unlikely event that a leap second is announced with an
#	effective date other than 30 June or 31 December, then this
#	file will be edited to include that leap second as soon as it is
#	announced or at least one month before the effective date
#	(whichever is later).
#	If an announcement by the IERS specifies that no leap second is
#	scheduled, then only the expiration date of the file will
#	be advanced to show that the information in the file is still
#	current -- the update time stamp, the data and the name of the file
#	will not change.
#
#	Updated through IERS Bulletin C60
#	File expires on:  28 June 2021
#
#@	3833827200
#
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
#
#	the following special comment contains the
#	hash value of the data in this file computed
#	use the secure hash algorithm as specified
#	by FIPS 180-1. See the files in ~/pub/sha for
#	the details of how this hash value is
#	computed. Note that the hash computation
#	ignores comments and whitespace characters
#	in data lines. It includes the NTP values
#	of both the last modification time and the
#	expiration time of the file, but not the
#	white space on those lines.
#	the hash line is also ignored in the
#	computation.
#
#h 	064356a8 39268b92 76e4d5ef 3e22fae1 0cca529c

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Editing: leap-seconds

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