This app was mentioned in 2 comments, with an average of 2.00 upvotes
I use Time Since. I like that it times smaller time intervals, since I struggle to go a day without picking. It's pretty simple and has a simple widget too.
I also used to use the MotivAider app with the Autistic kids I did behavioural therapy with. I don't seem to be able to find it. However, if you can get your hands on one of these it could be just what you're looking for. You can set it to vibrate automatically at a certain interval, so you know to give yourself a reward. They're also easy to reset, so really easy to use. I would buy one myself, but they're only available in the US.