Marciek,

Stop Watches usually requires precision greater than the interval timed. So external counters are a simple approach.

However, you can make a good Real Time Clock using a Watchdog Timer.

I have implemented Calendar/Clocks using the Dallas '320 Watchdog.

Yes, a trick or to is required to keep the long term accuracy in place. You have to implement assymetric counters where you use an alternate count for every other second.

It has one advantage it allows a WatchDog Time out of 1 second which is alot easier to margin into executing code than the shorter WD timeouts.

regards,
p