Replace the leap-seconds file in r320242 from USNO -

ftp://tycho.usno.navy.mil/pub/ntp/leap-seconds.3701462400 - with a
leap-seconds file from NIST at ftp://time.nist.gov/pub/. The USNO
version of the file changes the last documented leap second update
time whereas the NIST version does not. The expiration of the USNO
version of the file is also one month short.

Requested by:	ian@
Obtained from:	ftp://time.nist.gov/pub/leap-seconds.3676924800
MFC after:	3 days

etc/ntp/leap-seconds

@@ -15,12 +15,17 @@
 #	are transmitted by almost all time services.
 #
 #	The first column shows an epoch as a number of seconds
-#	since 1900.0 and 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.
+#	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
@@ -42,11 +47,11 @@
 #	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.fr for more information.
+#	(BIPM). See www.bipm.org for more information.
 #
-#	3. The current defintion of the relationship between UTC 
+#	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 than epoch, and it can be 
+#	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:
@@ -58,21 +63,19 @@
 #		of Time," Proc. of the IEEE, Vol. 79, pp. 894-905,
 #		July, 1991.
 #
-#	4.  The insertion of leap seconds into UTC is currently the
-#	responsibility of the International Earth Rotation Service,
-#	which is located at the Paris Observatory: 
+#	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.)
 #
-#	Central Bureau of IERS
-#	61, Avenue de l'Observatoire
-#	75014 Paris, France.
+#	Leap seconds are announced by the IERS in its Bulletin C.
 #
-#	Leap seconds are announced by the IERS in its Bulletin C
+#	See www.iers.org for more details.
 #
-#	See hpiers.obspm.fr or www.iers.org for more details.
-#
-#	All national laboratories and timing centers use the
-#	data from the BIPM and the IERS to construct their
-#	local realizations of UTC.
+#	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
@@ -102,7 +105,7 @@
 #
 #	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 equivlent to 
+#	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:
 #
 #	...
@@ -112,7 +115,10 @@
 #	...
 #
 #	in both cases the use of timestamps based on TAI produces a smooth
-#	time scale with no discontinuity in the time interval.
+#	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
@@ -121,14 +127,23 @@
 #	with, since the difficulty of unambiguously representing the epoch
 #	during the leap second does not arise.
 #
-#	Questions or comments to:
-#		Jeff Prillaman
-#		Time Service Department
-#		US Naval Observatory
-#		Washington, DC
-#		jeffrey.prillaman@usno.navy.mil
+#	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.
 #
-#	Last Update of leap second values:  18 Apr 2017
+#	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
@@ -136,7 +151,22 @@
 #	be identified by the unique pair of characters in the first two
 #	columns as shown below.
 #
-#$	 3701462400
+#$	 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
@@ -149,10 +179,11 @@
 #	is announced.
 #
 #	The following entry specifies the expiration date of the data
-#	in this file in units of seconds since 1900.0.  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
+#	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
@@ -168,10 +199,10 @@
 #	current -- the update time stamp, the data and the name of the file
 #	will not change.
 #
-#	Updated through IERS Bulletin C 53
-#	File expires on:  1 Dec 2017
+#	Updated through IERS Bulletin C53
+#	File expires on:  28 December 2017
 #
-#@	3721075200
+#@	3723408000
 #
 2272060800	10	# 1 Jan 1972
 2287785600	11	# 1 Jul 1972
@@ -205,7 +236,7 @@
 #	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 ~/sha for
+#	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
@@ -216,5 +247,4 @@
 #	the hash line is also ignored in the
 #	computation.
 #
-#h	3f004255 91f969f7 252361e5 27aa6754 eb6b7c72
-#
+#h	62cf8c5d 8bbb6dcc c61e3b56 c308343 869bb80d