Remind is a full-featured calendar/reminder program featuring sophisticated date calculation, moon phases, sunrise/sunset, Hebrew calendar, alarms, PostScript output, an X front-end, multilingual messages, and proper handling of holidays. It also includes scripts for making a nice WWW calendar server.
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.
Email-reminder allows users to define events that they want to be reminded of by email. Possible events include birthdays, anniversaries, weekly, monthly, and yearly events. Reminders can be sent on the day of the event and a few days beforehand. It uses a cron job to checks for events and send reminders once a day, and a simple GUI that allows users to edit the reminders they want to receive.
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.