If you use the number format, pay careful attention to what forestview typed and what you typed.

h:mm:ss or h:mm will give "time of day" where the display will be 0 to 24 hours depending on the time of day.
[h]:mm:ss or [h]:mm will give "elapsed time" where the display will be from 0 to whatever Excel's maximum allowed elapsed hours is (over 10E6 hours, by my tests).

guidelines for custom number formats (including time formats): https://support.office.com/en-us/art...7-9c9354dd99f5