Most phones have a countdown timer and stopwatch app built into the clock app, but this one is really an improvement if you need to keep track of multiple things. In my case I used it when I broke my leg to keep track of how long it had been since I took "as needed' pills and also to remind me of those that had to be taken on a fixed schedule.
Here it shows just 4 stopwatches, but it supports an unlimited number (as well as countdown timers), and you can give each a descriptive name. It does have ads, but almost all of the time it's just a small banner across the top. There is an add-free version, but the dev wants $4 which seems a bit much.
Download on google play