It's the other way round but I use a time timer[1] to give me a visual indication of how much time is left. I usually put around 40 min & extend by 15~30min if I want to focus more. It doesn't have a big clicky button, but I find physically rotating the dial satisfying.
I've also experimented with iOS timer to stop playing music after the set time is up and that worked well as a non intrusive reminder too.
Yeah, there's a thousand options for counting down, but there's nothing that counts up while ensuring that you're still on task. I'm sure there are some complicated apps that half work[0], but I want a physical device. I want to occasionally press an oh-so-satisfying physical button to attest that I'm still on task. This also solves the problem of forgetting and leaving the timer running, which destroys your record of how long you've worked. With my proposed device, the timer will stop by itself if you forget about it.
[0]: I say "half work" because an app can't ensure you're on task.
Hmm yeah I can see the appeal of something like that. I just had a look in my local Amazon and saw this study timer with a nice physical button which seems to have "Count up" functionality [1][2]. But it doesn't seem to have a feature to alert you while counting up.
I've also experimented with iOS timer to stop playing music after the set time is up and that worked well as a non intrusive reminder too.
[1] https://www.timetimer.com/