Add or Subtract Time
Add or subtract hours, minutes, and seconds from a time value. Useful for scheduling and time tracking.
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How to use this calculator
- 1
Enter your start time in hours, minutes, and seconds.
- 2
Select Add or Subtract.
- 3
Enter the duration to add or subtract.
- 4
The result shows the new time in both 24-hour and 12-hour format.
Frequently asked questions
How does this tool add or subtract a duration?
It converts the start time and the duration into seconds, adds or subtracts them, and then wraps the answer back onto a 24-hour clock. Nine thirty plus 2 hours 45 minutes is 12:15:00, shown as 12:15:00 PM in the 12-hour line. Each hour is 3,600 seconds and each day is 86,400 seconds in this tool.
How does it show that the result is on another day?
A day offset is appended to the time. Nine o’clock plus 30 hours gives 15:00:00 (+1 day), and 01:15 minus 3 hours 40 minutes gives 21:35:00 (-1 day). The tool does not know any calendar date, so the offset is only a count of days away from the start day.
What happens when the result lands exactly on midnight?
It is shown as 00:00:00 and 12:00:00 AM. Subtracting one second from 00:00:00 gives 23:59:59 (-1 day), and adding 45 seconds to 23:59:30 gives 00:00:15 (+1 day). Subtracting 60 hours from 12:00:00 gives 00:00:00 (-2 days).
Do I need to normalise the duration first?
No. Minutes and seconds can exceed 59. Entering 0 hours 90 minutes 90 seconds is the same as 1 hour 31 minutes 30 seconds, and the Duration line shows it in that normalised form: adding it to 09:30:00 gives 11:01:30.
Does it check that the start time is valid?
Not strictly. Entries are rounded to whole numbers, and the arithmetic then simply runs on them, so a start of 25 hours would be treated as one hour past a full day. The Start Time line echoes your rounded inputs, so check it matches what you intended, and keep hours between 0 and 23.
Does it account for time zones or daylight saving?
No. It is clock arithmetic: a day always has 86,400 seconds and no clock changes are applied. If the interval crosses a daylight saving change, the real elapsed time will differ from the clock difference by an hour. For travel across zones, add or subtract the zone difference as hours yourself.
Can I find the gap between two times instead?
This page applies a duration to a time. To find how long it is from one time to another, use the hours calculator on this site, which takes a start, an end and a break and returns the duration.
Add or subtract time — clock arithmetic with day roll-over
Why 60 and 24 break ordinary addition
Clock times do not behave like decimals, because minutes roll over at 60 and hours at 24. Adding 9:45 and 2:30 as plain numbers gives 11:75, which is not a time; the correct answer is 12:15. The calculator avoids the problem by converting everything to seconds first, doing a single addition or subtraction, and then converting back. Because the arithmetic happens on one total, a result that crosses midnight, or goes back past it, is handled without special cases, and the day offset reports how far the answer has moved.
Worked examples
Seven additions and subtractions were run through the calculator, using the start times, durations and operations listed in the table. They include the default case, two results that cross midnight forwards, two that cross it backwards, one that jumps two days, and a duration entered with oversized minutes and seconds.
| Start | Operation | Duration entered | Result | 12-hour form |
|---|---|---|---|---|
| 09:30:00 | add | 2 h 45 min | 12:15:00 | 12:15:00 PM |
| 09:00:00 | add | 30 h | 15:00:00 (+1 day) | 3:00:00 PM (+1 day) |
| 23:59:30 | add | 45 sec | 00:00:15 (+1 day) | 12:00:15 AM (+1 day) |
| 01:15:00 | subtract | 3 h 40 min | 21:35:00 (-1 day) | 9:35:00 PM (-1 day) |
| 00:00:00 | subtract | 1 sec | 23:59:59 (-1 day) | 11:59:59 PM (-1 day) |
| 12:00:00 | subtract | 60 h | 00:00:00 (-2 days) | 12:00:00 AM (-2 days) |
| 09:30:00 | add | 0 h 90 min 90 sec | 11:01:30 | 11:01:30 AM |
Reading the day offset
The offset is measured from the day you started on. Adding 30 hours to 09:00 is one full day plus 6 hours, so the answer is 15:00 on the next day. Subtracting 3 hours 40 minutes from 01:15 passes midnight and lands at 21:35 on the previous day. A duration that is an exact multiple of 24 hours leaves the time of day unchanged and only moves the offset. If you need the actual calendar date, apply the offset to your own date, or use a date calculator.
Typical jobs for it
Shift end times, back-to-back meeting plans, cooking and baking schedules, and flight or train arrivals are all start-plus-duration problems. Subtraction works for the reverse question, such as what time to leave to arrive by a deadline, or when a recording began if you know where it ended and how long it ran. For long-haul journeys, convert to a single time zone first, because the tool cannot apply zone differences.
Assumptions to keep in mind
The tool treats a day as exactly 86,400 seconds and every entry as a whole number, rounding decimals. It does not read dates or time zones and does not apply daylight saving changes, so a result that spans a clock change can be an hour out compared with the wall clock in your location. Enter hours as 0–23 for the start time, and use the operation selector for direction rather than negative durations.
Learn more from an authoritative source:
WikipediaWeeks Between Dates
Calculate the exact number of weeks (and remaining days) between any two dates.
Months Between Dates
Calculate the exact number of months and days between two dates.
Hours Calculator
Calculate total hours and minutes between two times, or sum multiple time entries for timesheets.
Years Between Dates
Calculate the exact number of years, months, and days between two dates.
Age Calculator
Calculate your exact age in years, months, and days from your date of birth.
Date Calculator
Add or subtract days, weeks, months, or years to any date to find a past or future date.
Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →