There's a good article about computerized timekeeping in the current issue of Government Computer News:
http://www.gcn.com/print/27_15/46509-1.html Who's got the time? As computer systems become increasingly precise, a conflict emerges between human and machine measurements By Joab Jackson To explain epoch time, Mr. Jackson writes: For Posix-based computers such as Unix or Linux, time began Jan, 1, 1970, at midnight. The time according to those machines is the number of seconds that have accumulated since. For instance, the Greenwich Mean Time 8:22 p.m., Tuesday, June 10, 2008, translates to 1213129345 in Posix-speak. Naturally, I fired up DateTime to verify this value: C:\>perl -MDateTime -le "$dt = DateTime->new( year => 2008, month => 6, day => 10, hour => 20, minute => 22 ); print $dt->epoch()" 1213129320 C:\> Hmm...off by 25 seconds. When I factor out DateTime, I get the same result: C:\>perl -MTime::Local -le "print timegm(0,22,20,10,5,2008)" 1213129320 C:\>perl -MPOSIX -le "print POSIX::strftime(q{%Y-%m-%dT%H:%M:%S}, gmtime(1213129345))" 2008-06-10T20:22:25 C:\>perl -MPOSIX -le "print POSIX::strftime(q{%Y-%m-%dT%H:%M:%S}, gmtime(1213129320))" 2008-06-10T20:22:00 C:\> There had been 23 leap seconds as of June 10, not 25, so this doesn't account for the difference exactly: C:\>perl -MDateTime -le "$dt = DateTime->new( year => 2008, month => 6, day => 10, hour => 20, minute => 22, time_zone => 'UTC' ); print $dt->leap_seconds()" 23 C:\> I've written the author to thank him for his excellent article and to ask him how he computed the value 1213129345. -- Jim Monty <html> <body> <p>***********************************************************************</p> <p>The information in this email is confidential and may be legally privileged. It is intended solely for the addressee. Access to this email by anyone else is unauthorized. If you are not the intended recipient, any disclosure, copying, distribution or any action taken or omitted to be taken in reliance on it, is prohibited and may be unlawful. When addressed to our clients any opinions or advice contained in this email are subject to the terms and conditions expressed in the governing KPMG client engagement letter.</p> <p>***********************************************************************</p> </body> </html>