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.