ExamClock
A full-screen clock that just displays the time, but starts countdown 30 minutes before end time. I use it primarily as a tool while administering exams.
In order to avoid stress, the countdown timer is not visible until the end of the interval approaches. The total countdown time and the time from which countdown is visible is settable in code.
This project started as a small Tkinter learning exercise, so there is a lot of polishing that still needs to happen. The basic construct works fairly well, however.
Combine with something like Caffeine or Mouse Jiggler to make sure your screensaver doesn't kick in.
Contact me at kees@leune.org if you have questions.