256 lines
10 KiB
Plaintext
256 lines
10 KiB
Plaintext
#
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# In the following text, the symbol '#' introduces
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# a comment, which continues from that symbol until
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# the end of the line. A plain comment line has a
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# whitespace character following the comment indicator.
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# There are also special comment lines defined below.
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# A special comment will always have a non-whitespace
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# character in column 2.
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#
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# A blank line should be ignored.
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#
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# The following table shows the corrections that must
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# be applied to compute International Atomic Time (TAI)
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# from the Coordinated Universal Time (UTC) values that
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# are transmitted by almost all time services.
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#
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# The first column shows an epoch as a number of seconds
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# since 1 January 1900, 00:00:00 (1900.0 is also used to
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# indicate the same epoch.) Both of these time stamp formats
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# ignore the complexities of the time scales that were
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# used before the current definition of UTC at the start
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# of 1972. (See note 3 below.)
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# The second column shows the number of seconds that
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# must be added to UTC to compute TAI for any timestamp
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# at or after that epoch. The value on each line is
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# valid from the indicated initial instant until the
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# epoch given on the next one or indefinitely into the
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# future if there is no next line.
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# (The comment on each line shows the representation of
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# the corresponding initial epoch in the usual
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# day-month-year format. The epoch always begins at
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# 00:00:00 UTC on the indicated day. See Note 5 below.)
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#
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# Important notes:
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#
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# 1. Coordinated Universal Time (UTC) is often referred to
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# as Greenwich Mean Time (GMT). The GMT time scale is no
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# longer used, and the use of GMT to designate UTC is
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# discouraged.
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#
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# 2. The UTC time scale is realized by many national
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# laboratories and timing centers. Each laboratory
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# identifies its realization with its name: Thus
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# UTC(NIST), UTC(USNO), etc. The differences among
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# these different realizations are typically on the
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# order of a few nanoseconds (i.e., 0.000 000 00x s)
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# and can be ignored for many purposes. These differences
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# are tabulated in Circular T, which is published monthly
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# by the International Bureau of Weights and Measures
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# (BIPM). See www.bipm.org for more information.
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#
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# 3. The current definition of the relationship between UTC
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# and TAI dates from 1 January 1972. A number of different
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# time scales were in use before that epoch, and it can be
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# quite difficult to compute precise timestamps and time
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# intervals in those "prehistoric" days. For more information,
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# consult:
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#
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# The Explanatory Supplement to the Astronomical
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# Ephemeris.
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# or
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# Terry Quinn, "The BIPM and the Accurate Measurement
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# of Time," Proc. of the IEEE, Vol. 79, pp. 894-905,
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# July, 1991. <http://dx.doi.org/10.1109/5.84965>
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# reprinted in:
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# Christine Hackman and Donald B Sullivan (eds.)
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# Time and Frequency Measurement
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# American Association of Physics Teachers (1996)
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# <http://tf.nist.gov/general/pdf/1168.pdf>, pp. 75-86
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#
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# 4. The decision to insert a leap second into UTC is currently
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# the responsibility of the International Earth Rotation and
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# Reference Systems Service. (The name was changed from the
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# International Earth Rotation Service, but the acronym IERS
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# is still used.)
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#
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# Leap seconds are announced by the IERS in its Bulletin C.
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#
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# See www.iers.org for more details.
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#
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# Every national laboratory and timing center uses the
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# data from the BIPM and the IERS to construct UTC(lab),
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# their local realization of UTC.
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#
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# Although the definition also includes the possibility
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# of dropping seconds ("negative" leap seconds), this has
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# never been done and is unlikely to be necessary in the
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# foreseeable future.
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#
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# 5. If your system keeps time as the number of seconds since
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# some epoch (e.g., NTP timestamps), then the algorithm for
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# assigning a UTC time stamp to an event that happens during a positive
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# leap second is not well defined. The official name of that leap
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# second is 23:59:60, but there is no way of representing that time
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# in these systems.
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# Many systems of this type effectively stop the system clock for
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# one second during the leap second and use a time that is equivalent
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# to 23:59:59 UTC twice. For these systems, the corresponding TAI
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# timestamp would be obtained by advancing to the next entry in the
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# following table when the time equivalent to 23:59:59 UTC
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# is used for the second time. Thus the leap second which
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# occurred on 30 June 1972 at 23:59:59 UTC would have TAI
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# timestamps computed as follows:
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#
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# ...
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# 30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds
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# 30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds
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# 1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds
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# ...
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#
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# If your system realizes the leap second by repeating 00:00:00 UTC twice
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# (this is possible but not usual), then the advance to the next entry
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# in the table must occur the second time that a time equivalent to
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# 00:00:00 UTC is used. Thus, using the same example as above:
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#
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# ...
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# 30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds
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# 30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds
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# 1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds
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# ...
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#
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# in both cases the use of timestamps based on TAI produces a smooth
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# time scale with no discontinuity in the time interval. However,
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# although the long-term behavior of the time scale is correct in both
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# methods, the second method is technically not correct because it adds
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# the extra second to the wrong day.
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#
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# This complexity would not be needed for negative leap seconds (if they
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# are ever used). The UTC time would skip 23:59:59 and advance from
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# 23:59:58 to 00:00:00 in that case. The TAI offset would decrease by
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# 1 second at the same instant. This is a much easier situation to deal
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# with, since the difficulty of unambiguously representing the epoch
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# during the leap second does not arise.
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#
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# Some systems implement leap seconds by amortizing the leap second
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# over the last few minutes of the day. The frequency of the local
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# clock is decreased (or increased) to realize the positive (or
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# negative) leap second. This method removes the time step described
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# above. Although the long-term behavior of the time scale is correct
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# in this case, this method introduces an error during the adjustment
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# period both in time and in frequency with respect to the official
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# definition of UTC.
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#
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# Questions or comments to:
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# Judah Levine
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# Time and Frequency Division
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# NIST
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# Boulder, Colorado
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# Judah.Levine@nist.gov
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#
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# Last Update of leap second values: 8 July 2016
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#
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# The following line shows this last update date in NTP timestamp
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# format. This is the date on which the most recent change to
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# the leap second data was added to the file. This line can
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# be identified by the unique pair of characters in the first two
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# columns as shown below.
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#
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#$ 3676924800
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#
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# The NTP timestamps are in units of seconds since the NTP epoch,
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# which is 1 January 1900, 00:00:00. The Modified Julian Day number
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# corresponding to the NTP time stamp, X, can be computed as
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#
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# X/86400 + 15020
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#
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# where the first term converts seconds to days and the second
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# term adds the MJD corresponding to the time origin defined above.
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# The integer portion of the result is the integer MJD for that
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# day, and any remainder is the time of day, expressed as the
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# fraction of the day since 0 hours UTC. The conversion from day
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# fraction to seconds or to hours, minutes, and seconds may involve
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# rounding or truncation, depending on the method used in the
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# computation.
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#
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# The data in this file will be updated periodically as new leap
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# seconds are announced. In addition to being entered on the line
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# above, the update time (in NTP format) will be added to the basic
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# file name leap-seconds to form the name leap-seconds.<NTP TIME>.
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# In addition, the generic name leap-seconds.list will always point to
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# the most recent version of the file.
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#
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# This update procedure will be performed only when a new leap second
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# is announced.
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#
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# The following entry specifies the expiration date of the data
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# in this file in units of seconds since the origin at the instant
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# 1 January 1900, 00:00:00. This expiration date will be changed
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# at least twice per year whether or not a new leap second is
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# announced. These semi-annual changes will be made no later
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# than 1 June and 1 December of each year to indicate what
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# action (if any) is to be taken on 30 June and 31 December,
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# respectively. (These are the customary effective dates for new
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# leap seconds.) This expiration date will be identified by a
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# unique pair of characters in columns 1 and 2 as shown below.
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# In the unlikely event that a leap second is announced with an
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# effective date other than 30 June or 31 December, then this
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# file will be edited to include that leap second as soon as it is
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# announced or at least one month before the effective date
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# (whichever is later).
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# If an announcement by the IERS specifies that no leap second is
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# scheduled, then only the expiration date of the file will
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# be advanced to show that the information in the file is still
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# current -- the update time stamp, the data and the name of the file
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# will not change.
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#
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# Updated through IERS Bulletin C58
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# File expires on: 28 June 2020
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#
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#@ 3802291200
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#
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2272060800 10 # 1 Jan 1972
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2287785600 11 # 1 Jul 1972
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2303683200 12 # 1 Jan 1973
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2335219200 13 # 1 Jan 1974
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2366755200 14 # 1 Jan 1975
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2398291200 15 # 1 Jan 1976
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2429913600 16 # 1 Jan 1977
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2461449600 17 # 1 Jan 1978
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2492985600 18 # 1 Jan 1979
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2524521600 19 # 1 Jan 1980
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2571782400 20 # 1 Jul 1981
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2603318400 21 # 1 Jul 1982
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2634854400 22 # 1 Jul 1983
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2698012800 23 # 1 Jul 1985
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2776982400 24 # 1 Jan 1988
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2840140800 25 # 1 Jan 1990
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2871676800 26 # 1 Jan 1991
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2918937600 27 # 1 Jul 1992
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2950473600 28 # 1 Jul 1993
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2982009600 29 # 1 Jul 1994
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3029443200 30 # 1 Jan 1996
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3076704000 31 # 1 Jul 1997
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3124137600 32 # 1 Jan 1999
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3345062400 33 # 1 Jan 2006
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3439756800 34 # 1 Jan 2009
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3550089600 35 # 1 Jul 2012
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3644697600 36 # 1 Jul 2015
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3692217600 37 # 1 Jan 2017
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#
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# the following special comment contains the
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# hash value of the data in this file computed
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# use the secure hash algorithm as specified
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# by FIPS 180-1. See the files in ~/pub/sha for
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# the details of how this hash value is
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# computed. Note that the hash computation
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# ignores comments and whitespace characters
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# in data lines. It includes the NTP values
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# of both the last modification time and the
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# expiration time of the file, but not the
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# white space on those lines.
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# the hash line is also ignored in the
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# computation.
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#
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#h f28827d2 f263b6c3 ec0f19eb a3e0dbf0 97f3fa30
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