Did you grab the DateTime module and install it from CPAN (or ppm if you are using Windows and ActivePerl)? DateTime isn't a default module that comes with Perl, so you have to explicitly install it before you can "use" it.
What operating system do you have Perl installed on? If it is Windows, try the PPM utility that should have come with the installation. If not, use the cpan command line utility. Just open up a shell session and type 'cpan DateTime'.
Alternatively, you can get it manually from CPAN and install it yourself, http:://search.cpan.org.
If you're on Windows installing yourself is such a pain!
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.