Combining a timer wither a calendar sounds like an interesting idea! For almost ten years, I've been using my own app for a similar purpose (tracking work productivity and video games), basically a to do list with timer, plus basic filtration to analyze the data. The app is here: https://github.com/Klaster1/timer-5 .

I've been tinkering with something similar too. This is for timing checklists not individual items—so typically multiple steps that repeat from time to time: https://tusklist.com