How to use the calculator
The page has two parts.
Duration between two times. Enter a start time and an end time. Seconds are optional. Press Calculate to see the time in between. If the period covers more than one day, also fill in the start date and the end date; the calculator then counts every day in between.
Add or subtract time. Enter a clock time, choose Add or Subtract, and type the hours, minutes and seconds to move by. Empty boxes count as zero. The result is a new clock time, with a note such as "+1 day" when it falls on a different day.
How the duration is worked out
Every time is turned into seconds after midnight. The start is subtracted from the end, and the difference is split back into hours, minutes and seconds:
- Total minutes = total seconds ÷ 60
- Decimal hours = total seconds ÷ 3,600, rounded to 2 decimals
When you leave the dates empty and the end time is earlier than the start time, the end is taken to be on the next day, and the result says so. This is what you want for a night shift or a flight that lands after midnight. When dates are given, nothing is assumed: an end that comes before the start is reported as an error.
Worked examples
- From 9:00 to 17:30 is 8 hours, 30 minutes and 0 seconds. That is 510 minutes, 30,600 seconds or 8.5 decimal hours.
- From 22:00 to 06:15 with no dates is 8 hours and 15 minutes, because 06:15 is read as the next morning.
- 22:30 plus 3 hours 45 minutes is 02:15 (2:15 AM), +1 day.
Good to know
- Decimal hours are not minutes. 8.5 hours means 8 hours and 30 minutes, not 8 hours and 50 minutes. Timesheets and invoices often need the decimal form; to convert, divide the minutes by 60 and add the result to the hours.
- Same start and end. Equal times without dates give a duration of zero. For a full 24 hours, enter dates one day apart.
- Daylight saving time is not applied. Every day is treated as exactly 24 hours long. On the night the clocks change, the real elapsed time can be one hour more or less than shown.
- Large values are fine. In the add or subtract section you can enter more than 59 minutes or seconds, and more than 24 hours; for example adding 90 minutes is the same as adding 1 hour 30 minutes. The day note then shows how many days later or earlier the result falls.