The daily-limit clock often lags behind by half an hour or more at the end of a workday.

I turned off micro-breaks (they are set to be included as work activity) and set 10min coffee-breaks at 1 hour intervals. The daily-limit itself is set to 8 hours.  The daily limit timer keeps on counting during coffee-breaks, as intended.

For example, today, the workrave clock started running at 07:55, system time. At the time of writing, 3h50m later, the daily-limit timer shows 4h27m remaining, whereas it should be 8h00-3h50=4h10m remaining.
Thus, there is a 17 minute lag already!

This problem is especially important for this type of application, because people that trust the WorkRave clock will spend too much time at the computer.

Interesting observation (somehow I didn't see this bug earlier...)

The daily limit timer should stop during restbreaks, Assuming 3 breaks in 3:50 of 8 minutes = 24 minutes, I think Workrave counted 7 minutes too much... Not sure why. Perhaps you postponed a break? or ignored a few warning windows?

Each time a timer starts or stops its 'active' time is truncated to whole seconds. So yes, you can lose up to 1 second each time a timer is started and stopped.  Perhaps I will change the timers to use a higher precision in the future.