a duration can be obtained by subtracting two time values (which are technically the date and the time). durations are specified as:
weeks, days, hours, minutes, seconds and milliseconds.
I do not know what you are trying to figure out...
- how long did the worker work? or
- when did the shift end?
can you state a question (without regard for the solution) so we can assist? Also a screen shot of what you enter and what you want as output would be helpful.
All that said you can subtract two times to get a duration. You can add durations to get another duration. You cannot add times. To specify that an entry is a time you enter in the format "0w 0d 30h 14m 22s 0ms" any of the entries with 0 may be omitted.
you can add durations in cells by using the "+" or "-" operator
as in A1-B1
where A1 contains "3h 45m" and B1 contains "1h 15m"
the result will be "2h 30m"
In Numbers, a Date-And-Time format is exactly that. A Date has a Time attached, and a Time has a Date attached. The cell format can display either the Date or the Time or both. A Duration is a difference between two events and the cell can display any period of difference, from weeks to milliseconds.
Here's your example, revised to work correctly:
"20:00+06:15=26:15)-24:00)=02:15am the next day."
Aug 13, 2013 20:00 + 6h 15m = Aug 14, 2013 02:15
Row 2 shows the full date and time entry in column A, the duration of the shift (as entered) in column B and the full result of the addition formula in column C.
Row 6 shows the same values and result.
Columns A and C have been formatted as Date and Time, Date: 'none', Time: 19:08
Column B has been formatted as Duration: 0:00