I want to create a very simple CGI script

CGI? Why? This alias works for me.

alias unixtime="perl -MTime::ParseDate -le'print parsedate(\"@ARGV\", 
+ZONE=>\"GMT\")'"
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Call it like:

$ unixtime Thu Feb  8 15:28:00 2007
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or

$ unixtime 2/8/07 3:28pm
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or

$ unixtime 2-8-2007 15:28
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or

$ unixtime `date`
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or ... well, you get the idea. It's pretty good at figuring out what you mean.