Time calculator: add and subtract time

Why adding time is tricky

Ordinary numbers carry over at 10, but time carries over at 60: 60 seconds make a minute and 60 minutes make an hour. That’s why 0:45 + 0:30 is 1:15 and not 0:75, and why a spreadsheet can give strange answers if times are typed as plain numbers. The calculator adds every row in seconds and only then turns the total back into hours, minutes and seconds, so the carrying is always right.

Here’s a playlist of three videos, 12:34, 8:51 and 23:07 long (minutes and seconds). The seconds add up to 34 + 51 + 7 = 92, which is 1 minute 32 seconds. The minutes add up to 12 + 8 + 23 = 43, plus the 1 carried over, so the playlist runs 44 minutes 32 seconds. Enter each video as a row with minutes and seconds and you get 0:44:32.

Subtracting and borrowing

Subtracting works the same way in reverse: when there aren’t enough seconds or minutes, you borrow 60 from the next column. Say a runner’s chip time was 4:12:35 but they crossed the start line 6 minutes 48 seconds after the gun. 35 seconds minus 48 doesn’t go, so borrow a minute: 95 − 48 = 47 seconds, then 11 − 6 = 5 minutes, leaving 4 hours. Their net time is 4:05:47. In the calculator, enter 4:12:35 as one row, then 0:06:48 as a second row set to −.

Decimal hours and minutes

Invoices, payroll and many spreadsheets want time as a decimal. Divide the minutes by 60 (and the seconds by 3,600) and add them to the hours. Some common ones:

Watch out for the classic mix-up: 7.30 in decimal is 7 hours 18 minutes, not 7 hours 30 minutes. If you bill by the hour, round the decimal the way your contract says, usually to two places.

Adding time to a clock time

A clock wraps around at midnight, so adding hours to a time of day can land on the next day. 9:45 PM plus 3 hours 20 minutes is 1:05 AM the next day, and 6:15 AM minus 8 hours is 10:15 PM the day before. Switch to From a clock time for these: the answer shows the new time, the 24-hour version and how many days it moved. It counts plain clock hours, so it doesn’t adjust for time zones or the hour that daylight saving time adds or removes.

For the hours between two clock times, like a shift from 8:45 AM to 5:15 PM, the hours calculator is quicker, and the time card calculator adds up a whole week with overtime. If your times are written like 1730, the military time converter turns them into AM and PM. To count days rather than hours, use days from today.