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You can work out that your 25 minute timer actually took 30 minutes this way, but you can't fix it to make it actually take 25 minutes.

HTTP 203 has a great video about counters (https://www.youtube.com/watch?v=MCi6AZMkxcU)which goes into more details about why counters/timers are a bit more complex.

Yeah, but I think putting it in the worker was about making the timer able to “go off” at a specific time, while the page is backgrounded.