Add or subtract durations, or see what time it will be before or after a known clock time.
How to use the Add Time Calculator
- To total durations: Choose Add or Subtract for each row. Enter any combination of days, hours, minutes, and seconds. Select Calculate to see the total.
- To shift a clock time: Enter a known time, such as 23:45 or 11:45 PM. Add or subtract a duration and check the day offset below the result.
Worked examples
- Duration: 2 h 45 min + 3 h 30 min = 6 h 15 min. The 75 minutes carry into 1 h 15 min, for 22,500 seconds in total.
- Clock time: 23:45 + 30 min = 00:15 the next day. The calculation crosses midnight and reports a +1 day offset.
Formula and limits
For each duration row, multiply days by 86,400, hours by 3,600, and minutes by 60. Add the seconds, apply the row’s sign, and sum the rows. Clock mode divides the shifted time into a signed day offset and a clock reading within that day.
The exact result uses whole seconds. Decimal hours and minutes are rounded for display. A day is treated as 24 hours, so clock results do not account for time zones, daylight saving changes, or leap seconds.
More time calculators
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Frequently asked questions
Can I subtract durations?
Yes. Choose Subtract for any row you want to take away. If the total is below zero, one minus sign applies to the full duration.
Can the total be more than 24 hours?
Yes. Duration mode carries each complete 24 hours into a day. It keeps the full total in exact seconds.
How do decimal hours work?
Divide the total seconds by 3,600. For example, 6 h 15 min is 6.25 hours because 15 minutes is a quarter of an hour.
What happens if I cross midnight?
Clock mode gives the new clock time and the number of days forward or backward. For example, 23:45 plus 30 minutes is 00:15 the next day.
Calculator inputs and corrections
The calculator works in your browser. It does not send the values you enter to our server. Use Copy result to put the result on your clipboard.
If you find an error, use Contact to share the page URL and a sample that reproduces the result. Use sample values instead of personal or work records.