I have a fair bit of experience writing tests for concurrent code that uses timers on Go. We started with an open source test library ( https://pkg.go.dev/github.com/benbjohnson/clock ). It had a lot of problems. This was many years ago, looks like it's seen some maintenance since so maybe it's better? Then we tried to fix it. Fixed all the obvious bugs but still had a lot of problems using it in practice. It's not enough to just handle the calls without context of who is calling them in concurrent code. Then we switched to using gomock which ended up also being hard to use.
It's quite tricky is sort of the bottom line. It's not enough to just create fake time there's a lot more to it.
Clockwork has been mostly very good for me too. There are some sharp corners that I've run into periodically (e.g. it can't observe tickers, they're just an always-present Waiter) that I've been trying to figure out how to resolve in a new library, but overall I've been happier with clockwork than with any other I've seen so far.
Quartz looks quite interesting though. Gonna have to explore it in detail, because I agree with most of their claims in the article. I'm not 100% sold on their solutions being actual solutions and not just "often an improvement", but that'll probably be clear after using it for a bit / reading it in detail.
It's quite tricky is sort of the bottom line. It's not enough to just create fake time there's a lot more to it.