Check out Date::Calc for date calculations. I don't think it will go any more granular than days though.

Update
I notice that your date/time format is easily parseable. One thing you can do is use Date::Calc's Date_to_Time() function to convert that information into a system time format (aka seconds-since-epoch).

$time = Date_to_Time($year,$month,$day, $hour,$min,$sec);
[download]

You could then operate on that system time in a number of ways, including manual subtraction to find the difference in seconds. You should be able to see where to go from there. :)