Using time.sleep(1) is not accurate

[fusselkater]

As all modern OS implement some kind of multi tasking, when you call sleep, your process will be set to sleeping state until the sleep time has elapsed. But it will not reactivate your process exactly after 1 second, instead it will be 1 second + some more or less random x.
So this timer is not accurate especially for long durations.
The better way to implement this would be to save your current system time when you click start (e.g. with starttime = datetime.datetime.now()). Then while running, substract current time from starttime:

import datetime
starttime = datetime.datetime.now()

# later
delta = datetime.datetime.now() - starttime

delta will be a datetime.timedelta object with minutes/seconds/milliseconds that elapsed since starttime.

[Sonic2kDBS]

Hi, thank you for bringing that up. You are right. I didn't thought about that until you mentioned it.
I did some testing and there is indeed a time difference. The difference is about 1/2 Second on 10 minutes, so 3 seconds on an hour. For the full slider time of 120 minutes it is 6 seconds.

Thank you for your suggestions. I will use them in the next release to make the Desk-Timer more precise.