Back in 2013 I was looking for a job and I thought it'd be fun to crank out a quick interval timer to prove that I knew Android. All the timers I'd used in the play store (and there are literally dozens) were quick, dirty jobs with one or more terrible problems, and I wanted something that actually worked. The problem was that
A) Android, as I slowly learned, is a painfully treacherous and unproductive platform and B) If I wanted this to show that I wasn't a terrible developer I'd actually need to put in some effort to make it decent
As a result I didn't quite finish it in the next few weeks... in fact I basically only worked on it in bursts here or there and it stretched on for two years before I finally got inspired to finish the damn thing. You can find it on the play store at https://play.google.com/store/apps/details?id=com.alexgilleran.hiitme, or you can type "HIIT Me" into the play store search then scroll down to the 47th result :(.