Hey again Timo - think I've got something a bit more useful now.
When the work unit goes past midnight (whether or not it goes idle during that time doesn't seem to matter), and I click 'Stop' during the same session, it seems to work correctly.
However if the computer goes to sleep during a session - for example I close the lid without manually 'Stopping' the timer, then the next time it's opened up, the Idle kicks in and the timer starts counting again - but when I click 'Stop', everything past Midnight is counted as Zero seconds (even though the minutes will be, say, 00:00 - 10:00, and even if there had been activity during that time).
Let me know if that helps / if anything else I can try out to help.
thanks,
Danielle