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
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