The following has been kicking around my "Handling_Leapseconds directory for more than a year. It is a complete replacement for the current "leapseconds.awk".  It uses the NTP timestamp *data* rather than the *comments* at the end of each leap-second datum from the leap-seconds.list file. I found it again today and finished the changes needed to use Dennis' sstamp_to_ymdhMs() AWK function.  I've incorporated some AT&T UNIX programming style changes (which I learned back in the dark ages), too.  I'm sure Paul will re-arrange it whichever way he wants before (or even if) it makes it into the TZ distribution. #    -=*< This file is in the public domain !! >*=- # Generate zic LEAP commands from an NIST format 'leap-seconds.list'. BEGIN {   print "# This file is in the public domain\n"   print "# Allowance for leap-seconds added to each time zone file.\n"   print "# This script generates zic \"Leap\" directives from public-domain data"   print "# in the NIST format leap-seconds.list file (originally created for the"   print "# Network Time Protocol (NTP) daemon by Judah Levine).  Find it at:\n"   print "#    <ftp://ftp.nist.gov/pub/time/leap-seconds.list>"   print "# -or-    <ftp://ftp.boulder.nist.gov/pub/time/leap-seconds.list>\n"   print "# For more info about the leap-seconds.list file, please see:\n"   print "# The NTP Timescale and Leap Seconds by Prof. David L. Mills."   print "#    <https://www.eecis.udel.edu/~mills/leap.html>.\n"   print "# The rules for leap-seconds are specified in Annex 1 (Time scales) of:\n"   print "#   \"Standard-frequency and time-signal emissions\""   print "#   International Telecommunication Union -"   print "#   Radiocommunication Sector Recommendation (ITU-R) TF.460-6 (02/2002)\n"   print "#    <https://www.itu.int/rec/R-REC-TF.460-6-200202-I/>\n"   print "# which is incorporated by reference in the [ITU] Radio Regulations.\n"   print "# The International Earth Rotation and Reference Systems Service (IERS)"   print "# periodically uses leap seconds to keep UTC to within 0.9 s of UT1"   print "# (which is a proxy for the earth's angular orientation in space)"   print "# and publishes leap-second data in a copyrighted file"   print "#    <https://hpiers.obspm.fr/iers/bul/bulc/Leap_Second.dat>."   print "# USNO also has a comprehensive uncopyrighted list of leap-seconds at:"   print "#    <ftp://199.211.133.23/ser7/leapsec.dat> (AKA maia.usno.navy.mil)"   print "# Also see: Coordinated Universal Time and the leap second by Judah Levine"   print "# URSI Radio Sci Bull. 2016;89(4):30-6. doi:10.23919/URSIRSB.2016.7909995"   print "#    <https://ieeexplore.ieee.org/document/7909995>.\n"   print "# There were no leap-seconds before 1972, as no official mechanism existed"   print "# accounting for the discrepancy between atomic time (TAI) and UT1."   print "# All leap-seconds are Stationary (S) at the given UTC time.\n"   print "# The correction (+ or -) is made at the given time, so lines"   print "# will typically look like:\n"   print "#  Leap    YEAR    MON    DAY    23:59:60    +    S"   print "# - or -"   print "#  Leap    YEAR    MON    DAY    23:59:59    -    S\n"   monthabbr[ 1] = "Jan" ; monthabbr[ 2] = "Feb" ; monthabbr[ 3] = "Mar"   monthabbr[ 4] = "Apr" ; monthabbr[ 5] = "May" ; monthabbr[ 6] = "Jun"   monthabbr[ 7] = "Jul" ; monthabbr[ 8] = "Aug" ; monthabbr[ 9] = "Sep"   monthabbr[10] = "Oct" ; monthabbr[11] = "Nov" ; monthabbr[12] = "Dec"   # In case the input has CRLF form a la NIST   RS = "\r?\n"   old_TAI_minus_UTC = int( 0 ) # This variable should be initialize !!   sstamp_init() # Init function } # Note: maintaining case, spacing or punctuation can # not be depended upon when humans are involved... /^#[ \t]*[Uu]pdated through[ \t:]*/ || /^#[ \t]*[Ff]ile expires on[ \t:]*/ {     last_lines = last_lines $0 "\n" } /^#[$][ \t]/ { updated = strtonum( $2 ) } # Save update NTP timestamp /^#[@][ \t]/ { expires = strtonum( $2 ) } # Save expiry NTP timestamp /^[ \t]*#/ { next } {     NTP_timestamp = int( $1 )    # NTP epoch timestamp     TAI_minus_UTC = int( $2 )    # delta T     if ( old_TAI_minus_UTC ) {     if ( old_TAI_minus_UTC < TAI_minus_UTC ) {         sign = "23:59:60\t+"     } else {         sign = "23:59:59\t-" }     sstamp_to_ymdhMs( NTP_timestamp - 1, ss_NTP ) # Note minus one !!     printf "Leap\t%d\t%s\t%d\t%s\tS\n", ss_y, monthabbr[ ss_m ], ss_d, sign     }     old_TAI_minus_UTC = TAI_minus_UTC } END {     # The difference between the NTP and POSIX epochs is 70 years     # (including 17 leap days), each 24 hours of 60 minutes of 60     # seconds each.     epoch_minus_NTP = ((1970 - 1900) * 365 + 17) * 24 * 60 * 60     print ""     print "# POSIX timestamps for the data in this file:"     sstamp_to_ymdhMs( updated, ss_NTP )     printf "# Updated: %s (%02d %s %d)\n", ( updated - epoch_minus_NTP ), ss_d, monthabbr[ ss_m ], ss_y     sstamp_to_ymdhMs( expires, ss_NTP )     printf "# Expires: %s (%02d %s %d)\n", ( expires - epoch_minus_NTP ), ss_d, monthabbr[ ss_m ], ss_y     printf "\n%s", last_lines } # Contributed by # #     Dennis Ferguson <dennis.c.ferguson@gmail.com> on Sun, 14 Oct 2018 21:56:57 -0700 # # sstamp_to_ymdhMs() - convert (NTP, POSIX) seconds stamp to date and time # # Call as: # #    sstamp_to_ymdhMs( sstamp, epoch_days ) # # where: # #    sstamp - is a seconds timestamp, e.g. POSIX or NTP #    epoch_days - is the timestamp epoch in (proleptic Gregorian) days since #                 1 Mar 1600. ss_NTP is appropriate for a NTP sstamp, ss_POSIX #                 for a Unix sstamp. # # On return the following variables are set based on sstamp: # #    ss_y - calendar year for the sstamp (e.g. 2016) #    ss_m - month of the year (1-January to 12-December) #    ss_d - day of the month #    ss_h - hour (0-23) #    ss_M - minute (0-59) #    ss_s - second (0-59) #    ss_w - day of week (0-Sunday to 6-Saturday) # # The function # #    sstamp_init() # # should be called prior to using sstamp_to_ymdhMs(). # # The calendar computation part of this starts with a count of days since # March 1, 1600 (in a proleptic Gregorian calendar) and computes a ymd # date with days of the month numbered 0-30, months (March-February) numbered # 0-11 and years which run from March-February. The last four lines of code # in the function convert the date to a conventional day of month (1-31), # month (1-12, January-December) and Gregorian year. # # In the alternate calendar a year starts at March 1 and ends after the last # day of February. A quad-year starts on March 1 of a year evenly divisible # by 4 and ends after the last day of February 4 years later.  A century # starts on and ends before March 1 in a year evenly divisible by 100 # (i.e. 1600, 1900, 2100). A quad-century starts on and ends before March 1 # in years divisible by 400 (1600, 2000, 2400). Given this the magic constants # used here are: # #   days per year = 365 #   days per quad-year = 1461 = (365 * 4) + 1 #   days per century = 36524 = (1461 * 25) - 1 #   days per quad-century = 146097 = (36524 * 4) + 1 #   weekday (0-Sunday 6-Saturday) of first day of quad-century = 3 (Wednesday) # # Note that while the number of days in a quad-century is a constant the # number of days in each of the other time periods can vary by 1. In all # these cases, however, the variation is in the last day of the time period # (there might or might not be a February 29) where it is easy to deal with. # # The three standard day_epochs are: # #   ss_MJD   =  94493 #   ss_NTP   = 109513 #   ss_POSIX = 135080 function sstamp_init(){   ss_mon_days[ 1] = 31   ss_mon_days[ 2] = 61   ss_mon_days[ 3] = 92   ss_mon_days[ 4] = 122   ss_mon_days[ 5] = 153   ss_mon_days[ 6] = 184   ss_mon_days[ 7] = 214   ss_mon_days[ 8] = 245   ss_mon_days[ 9] = 275   ss_mon_days[10] = 306   ss_mon_days[11] = 337   ss_NTP = 109513   ss_POSIX = 135080 } function sstamp_to_ymdhMs( sstamp, epoch_days, ss_qcent, ss_cent, ss_qyr, ss_yr ){   ss_s = sstamp % 86400                 # isolate hms   ss_h = int( ss_s / 3600 )   ss_s %= 3600   ss_M = int( ss_s / 60 )   ss_s %= 60   ss_d = int( sstamp / 86400 ) + epoch_days  # select epoch   ss_qcent = int( ss_d / 146097 )   ss_y = 1600 + ss_qcent * 400   ss_d -= ss_qcent * 146097   ss_w = ( ss_d + 3 ) % 7   ss_cent = int( ss_d / 36524 )   if (ss_cent == 4) { ss_cent = 3  }   ss_d -= ss_cent * 36524   ss_y += ss_cent * 100   ss_qyr = int( ss_d / 1461 )   ss_y += ss_qyr * 4   ss_d -= ss_qyr * 1461   ss_yr = int(ss_d / 365)   if ( ss_yr == 4 ) { ss_yr = 3 }   ss_d -= ss_yr * 365   ss_y += ss_yr   for ( ss_m = 11 ; ss_m > 0 ; ss_m-- ){     if( ss_d >= ss_mon_days[ss_m] ){         ss_d -= ss_mon_days[ss_m] ;         break ;     }   }   # convert to conventional calendar   ss_d += 1;   if (ss_m > 9) {     ss_m -= 9 ; ss_y++   } else { ss_m += 3 } }