Numbers recognises date and time durations so you can use them in calculations. For example, say you want to calculate the total or average of your lap times in a race. You could enter a lap time as “2min 2sec” or any other commonly accepted units of time.
When you perform a calculation using these values, Numbers takes into account that there are 60 seconds in a minute, 24 hours in a day, and so on, so that the result you get is a proper duration of time.
To set how durations are displayed: