Date::Manip is a Perl module for handling Gregorian dates. Operations such as comparing two times, calculating a time a given amount of time from another, or parsing international times are all easily done. The library is oriented towards the type of operations people tend to think of rather than those operations used routinely by computers.
When is an extremely simple personal calendar program, aimed at the Unix geek who wants something minimalistic. It can keep track of things you need to do on particular dates. It's a very short and simple program, so you can easily tinker with it yourself. It doesn't depend on any libraries, so it's easy to install. You should be able to install it on any system where Perl is available, even if you don't have privileges for installing libraries. Its file format is a simple text file, which you can edit in your favorite editor.