Calc time difference between two date/time/zones

I have been searching and cannot find a way to determine the time
between two date/time/zones as shown below. Does anyone have a
routine that can do this easily without having to manually convert
everything to gmt? Thanks.

16 Oct 2008 07:57:01 -0500
16 Oct 2008 20:58:59 +0800
-----------------------------------------
= 4 seconds


15 Oct 2008 07:57:01 -0500
16 Oct 2008 01:58:59 +0800
-----------------------------------------
= 5 hrs 4 secs

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