Hi, Fratello

Even using ON INTERRUPT ... you shouldn't have such a time error ... ( just display " not so regularly " the correct HH:MM:SS )

but maybe you could write the shortest as possible interrupt stubb, not jumping from here to there ...

the problem looks to be your interrupt stubb is longer than the time between two Timer interrupts. ( 1/61 s ? - from your ISR calculations )

Alain