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Alternatively, you can get it manually from CPAN and install it yourself, http:://search.cpan.org.
PPM is brilliant:
Open a command prompt
Type 'ppm', it should find it in the path
At the ppm prompt type 'search datetime' and it should list the module
Type 'install datetime' and it will download and install it for you, and also any dependancies it has.
I've used timelocal in the past to convert dates/times to seconds and work out the difference in seconds between them.