How to Add and Subtract Time (Hours and Minutes)

Adding times: carrying over 60

Time arithmetic works like ordinary addition, except the minutes column rolls over at 60 instead of at 10. So you add the minutes, and if they reach 60 or more you subtract 60 and carry 1 into the hours column. This single rule handles almost every everyday addition of durations, from cooking times to work shifts.

Worked example: add 2:45 and 1:30. The minutes give 45 + 30 = 75, which is 60 or more, so subtract 60 to get 15 minutes and carry 1 hour. The hours give 2 + 1 + 1 (carried) = 4. The answer is 4:15. Notice how the carried hour is what keeps the result correct — forget it, and you would wrongly get 3:15.

Subtracting times: borrowing an hour

Subtraction uses the mirror image of the carry: borrowing. If the minutes you are subtracting are larger than the minutes you have, take one hour from the hours column and add it to the minutes as 60. Now the minutes are big enough to subtract from, and you continue as normal. This is exactly like borrowing in column subtraction, just with 60 instead of 10.

Worked example: 9:20 minus 45 minutes. You cannot take 45 from 20, so borrow one hour: 9 hours becomes 8 hours and the 20 minutes become 80. Now 80 − 45 = 35 minutes, and the hours are 8. The answer is 8:35. Borrowing turns an impossible subtraction into a straightforward one and prevents negative minutes.

When the result crosses midnight

Clock time wraps around every 24 hours, so adding a duration to a time of day can push you past midnight into the next day. If your hours total reaches 24 or more, subtract 24 and remember to advance the date by one day. For instance, 22:30 plus 3:00 gives 25:30, which becomes 1:30 the following day.

The same idea works backward: subtracting past 0:00 lands you on the previous day. These wrap-arounds are where manual time math most often trips people up, especially when a shift or a flight spans midnight. A time-duration calculator tracks the day change for you, so an overnight span shows the correct end date without any mental juggling.

Frequently asked questions

Why do minutes carry at 60 instead of 100?

Because an hour is defined as 60 minutes, not 100, so the minutes column fills up and rolls over at 60. This is why you cannot simply add times as if they were ordinary decimals. Always convert the overflow above 60 into one hour before reading off the result.

How do I add times that include seconds?

Extend the same rule to a third column: seconds carry into minutes at 60, and minutes carry into hours at 60. Add seconds first, carry into minutes, then add minutes, carry into hours. The chain of carries keeps everything consistent right up to the hours.

What if subtracting gives a negative result?

A negative result usually means you subtracted the later time from the earlier one, or that the span crosses midnight. Swap the order to get the positive duration, or add 24 hours if the end time is on the next day. A calculator handles both cases and reports the elapsed time correctly.

Can I add times written in the 12-hour format?

Yes, but convert to a 24-hour clock first to avoid AM/PM mistakes, then convert back at the end. For example, 11:30 PM is 23:30 on a 24-hour clock; do the arithmetic there and translate the final answer. This one conversion removes the most common source of errors around noon and midnight.