Constructed from WickedDevice's Day Counter Kit
Can operate in either counting up to 99, or can be set to count down from a set number and then trigger something else.
It's designed to run off USB power (5VDC), though of course every time you fully powered off your computer, it would reset.
As I wanted mine permanently mounted on my wall, I chopped the plug off a Nintendo DS charger and used the 5V's out of that as a power source.
As my sister is pretty good at breaking stuff, for Christmas I repurposed this, so that she can keep track of how many days have passed since a glass/plate/window has been broken.
I'm not sure what her current high score is, but it may well be into double figures.
Dirk and Victor (who run WickedDevice) are nice people, after I bought two of these kits and one of the IC's was duff, they where helpful in sending emails back and forth troubleshooting and in the end sent me a whole replacement kit. They also make some other fun micro-controller based kits as well.