According to The International Earth Rotation Service (IERS) we have "Leap Seconds" included in our UTC time.
Please refer http://hpiers.obspm.fr/iers/bul/bulc/bulletinc.dat . ~ snip ~ A positive leap second will be introduced at the end of June 2012. The sequence of dates of the UTC second markers will be: 2012 June 30, 23h 59m 59s 2012 June 30, 23h 59m 60s 2012 July 1, 0h 0m 0s ~snap~ The command 'date' doesn't calculate it. Test: $ date +%s -d "2012-06-30 23:59:60" date: invalid date `2012-06-30 23:59:60' Tested version: date (GNU coreutils) 8.5 Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>. This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Written by David MacKenzie. -- Juergen 'sysdef' Heine, gnu-d...@sysdef.de