Quite often when giving a course or a workshop I give participants a task to do. In order to let them control the time I wrote this little countdown timer. Over time it gained some more functionality besides ''raw'' time counting so by now I do not miss anything. Hopefully you will find it useful.
###TL;DR
Head to the out directory for comprehensive source code documentation in PDF, extracted Tcl code and an OSX executable application. If you want a Windows or a Linux version, let me know. It is quite easy to make and the only reason I haven't prepared it is that I cannot test it.
###Long version:
The timer.w is a literate program, written in nuweb. Literate programs are meant for humans (as opposed from computers) but they containg everything for both parties: easy to read documentation and full computer code.
From the literate source one can generate executable code (which, in this case will be Tcl) and LaTeX documentation which, in turn, can be rendered in PDF. If you want to look into the details you can find all these files in the out folder.