You might try this ...

$ cat datediff.pl 
#!/usr/bin/perl -w
use strict;
use Time::JulianDay;
use vars qw/ $opt_a $opt_b /;
use Getopt::Std;

my $dateA = "59:59:23:31:11:2002"; # 1 second before 2003
my $dateB = "00:00:00:01:00:2003"; # 2003

getopt( "a:b:");
if( $opt_a ) { $dateA = $opt_a; }
if( $opt_b ) { $dateB = $opt_b; }

print "dateA = $dateA\n";
print "dateB = $dateB\n";

my ($sec,$min,$hours,$mday,$month_0_to_11,$year)  = split /:/, $dateA;
print "($sec,$min,$hours,$mday,$month_0_to_11,$year)\n";
### ssA is the seconds since JAN 1st 1970 for dateA
my $ssA = jd_timelocal($sec,$min,$hours,$mday,$month_0_to_11,$year);
print "seconds from Jan 1st 1970 to 1 second before 2003 = $ssA\n";


($sec,$min,$hours,$mday,$month_0_to_11,$year)  = split /:/, $dateB;
print "($sec,$min,$hours,$mday,$month_0_to_11,$year)\n";
### ssB is the seconds since JAN 1st 1970 for dateB
my $ssB = jd_timelocal($sec,$min,$hours,$mday,$month_0_to_11,$year);
print "number of seconds from Jan 1st 1970 to midnight 2003 = $ssB\n";

my $diff = $ssB - $ssA;

print "The diffence in seconds between $dateB and $dateA = $diff\n";
[download]

Here's soe output ...

$ ./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 = 10414079
+99
(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
[download]

the module Time::JulianDay is very to install.