Time & dates
Add or subtract time
Choose a starting clock time and how many hours and minutes to add or take away; the result shows the new time and which day it lands on.
Hours can go above 24; the day line tells you how many days later or earlier the result falls.
How adding time to a clock time works
Clock times wrap around every 24 hours, so the trick is to work in minutes and then fold the result back into a day. Convert the starting time to minutes after midnight, add (or subtract) the hours × 60 plus the minutes, and divide by 1,440. The whole number of the division is the day shift and the remainder is the new clock time. When subtracting takes you below zero, the result belongs to the previous day.
Example: 08:45 plus 3 hours 30 minutes. The start is 525 minutes after midnight; adding 210 gives 735, which is 12 hours 15 minutes, so the answer is 12:15 on the same day. Add 18 h 30 min to 20:00 instead and you get 1,200 + 1,110 = 2,310 minutes; 2,310 − 1,440 = 870, so the result is 14:30 the next day. Subtracting 2 h 15 min from 01:00 gives 60 − 135 = −75 minutes, which is 22:45 on the previous day.
| Start | Change | Result | Day |
|---|---|---|---|
| 08:45 | +3 h 30 min | 12:15 | same |
| 14:20 | +45 min | 15:05 | same |
| 20:00 | +18 h 30 min | 14:30 | +1 |
| 01:00 | −2 h 15 min | 22:45 | −1 |
| 06:00 | +50 h | 08:00 | +2 |
Where it helps and what to watch
Typical uses are working out when a task, flight leg, cooking time or medication interval ends, when to set off to arrive somewhere on time (subtract the travel time from the arrival), or when a 36-hour deadline expires. For long additions, check the day line, not just the clock: 50 hours later looks like only 2 hours later on a clock face.
Mental arithmetic goes wrong mostly at the 60-minute boundary. People add 40 minutes to 10:35 and write 10:75, or take 50 minutes from 9:10 and get 8:60. Carry over carefully: 35 + 40 = 75 minutes, which is 1 hour 15 minutes, giving 11:15; and 9:10 minus 50 minutes is 8:20. The calculation assumes every day has 24 hours; on the night daylight saving time begins or ends, the real elapsed time can differ by an hour. For durations across many days, a date calculator is the better tool.
Frequently asked questions
What time will it be in 8 hours?
Add 8 to the current hour and subtract 24 if the result is 24 or more. At 19:30, 8 hours later is 27:30, which is 03:30 the next morning.
How do I subtract hours and minutes from a time?
Subtract the minutes first, borrowing 60 from the hours if needed, then subtract the hours. 10:20 minus 1 h 45 min: borrow to make 9:80, then 9:80 − 1:45 = 8:35.
Can I add more than 24 hours to a time?
Yes. Every full 24 hours moves the result one day later and leaves the clock time unchanged, so 06:00 plus 50 hours is 08:00 two days later.