Who can point me in the direction of a kind of stopwatch base?

Hello everyone,

I want to make an application like Google clock, and then especially the stopwatch part. Like it needs to continue in background and when app closes create an notification where you can stop and pause the time etc. I will figure that all out, but I don’t know how to start. I can only find countdown counters, but I need to count down. Who can tell me where I need to start to get this app working?

Thanks in advance,

With kind regards,