Excel stores date/time information as "days since 00 January 1900". If you subtract one from t'other, you get an answer in the same units. If you display it as date/time, then it appears as nonsense. If you try to add them when displayed in that way, then the total is even more nonsense.
Likewise if you just display time. That's because if you're displaying in hours and minutes, it will disregard whole days. Add 23:59 and 00:02 and you get 00:01 because that's what your clock face does.
However, if you display both as date time etc (however you prefer to see date and time), but calculate the difference in (say) hours, you can add them with no problem.
I made a little Excel sheet that shows how it works - it's
here.