Timing for an event longer than 24 hours

[NDBC]

I would like to set up a timer that can go over 24 hours. I currently have a
timer that uses now()-start time but this goes back to 0 after 24hrs. Is it
possible in Excel to get a timer that will show say 25:00:00 at the 25 hour
mark.

Thanks

 

[Niek Otten]

Format Custom as [hh]:mm:ss

 

[Bob Phillips]

format the cell as [h]:mm:ss