$ ./datediff.pl 
dateA = 59:59:23:31:11:2002
dateB = 00:00:00:01:00:2003
(59,59,23,31,11,2002)
number of seconds from Jan 1st 1970 to 1 second before 2003 = 1041407999
(00,00,00,01,00,2003)
number of seconds from Jan 1st 1970 to midnight 2003 = 1041408000
The diffence in seconds between 00:00:00:01:00:2003 and 59:59:23:31:11:2002 = 1