This took some figuring out! Set the cells that you will be using for the calculation in the custom [h]:mm:ss format from the drop-down menu at the top of the window. Enter the numbers, making sure to put 0’s if the value is blank.
i.e. 12 hours, 5 seconds = 12:00:05
Complete the SUM function to calculate the total of the times, making sure that the result cell is also in the [h]:mm:ss format.
Things that don’t work:
Subtracting. Normally you just use the SUM function and include negative numbers to subtract them, but excel won’t accept the negative times.
60+ minutes or seconds. It just freaks out and won’t do it. You can have over 12 or 24 hours, though.
Further reference: Microsoft support
If you’re not liking this honestly pretty bad tool, this one can subtract more easily: Browser Time Calculator