in reply to How do I find difference between two timestamps?

The timegm function in Time::Local can convert a Zulu (GMT) time into a timestamp. Since your date formats are nonstandard, it would be simplest to break out the year, month, day, hours, minutes, and seconds with a regular expression or substrings, then do:
$time = timegm($sec,$min,$hour,$mday,$mon,$year);
These times can be subtracted to give differences in seconds. You'll have to subtract the milliseconds separately and add that to the difference.