To add or subtract hours from a time, the easiest method is to convert to the 24-hour clock, do the arithmetic, then wrap around 24 if you pass midnight. For example, 8 hours after 7:00 PM is 19 + 8 = 27 → 27 − 24 = 3:00 AM the next day. And 8 hours before 5:00 AM is 5 − 8 = −3 → −3 + 24 = 9:00 PM the previous day.
Method 1: the 24-hour clock
- Convert the start time to 24-hour format: 3:30 PM → 15:30. (Our military time chart helps.)
- Add or subtract the hours and minutes.
- If the result is 24 or more, subtract 24 and move to the next day. If it's below 0, add 24 and move to the previous day.
- Convert back to AM/PM if you need to.
Worked examples
| Question | Working | Answer |
|---|---|---|
| 9:15 AM + 5 hours | 09:15 + 5 = 14:15 | 2:15 PM, same day |
| 10:00 PM + 6 hours | 22 + 6 = 28 → 28 − 24 = 4 | 4:00 AM, next day |
| 2:00 PM − 8 hours | 14 − 8 = 6 | 6:00 AM, same day |
| 3:00 AM − 8 hours | 3 − 8 = −5 → −5 + 24 = 19 | 7:00 PM, previous day |
| 11:40 AM + 2 h 35 min | 11:40 + 2:35 = 13:75 → 14:15 | 2:15 PM |
| 1:10 PM − 3 h 25 min | 13:10 − 3:25 → 12:70 − 3:25 = 9:45 | 9:45 AM |
Handling minutes
- Adding: if the minutes add up to 60 or more, subtract 60 and add 1 to the hour. 13:75 → 14:15.
- Subtracting: if you can't subtract the minutes, "borrow" an hour: turn 13:10 into 12:70, then subtract.
- Decimals: 1.5 hours is 1 hour 30 minutes, not 1 hour 50 minutes. Multiply the decimal by 60: 0.25 h = 15 min, 0.75 h = 45 min.
Method 2: count to the next landmark
If you prefer staying on the 12-hour clock, count to the nearest "landmark" (noon or midnight), then add the rest:
- 9:00 PM + 7 hours: 3 hours gets you to midnight, and 4 more makes 4:00 AM.
- 10:30 AM − 12 hours: 12 hours is a half-turn of the clock, so the time stays the same but AM switches to PM: 10:30 PM the previous day.
Shortcuts for common amounts:
- ±12 hours: same clock time, AM ↔ PM swaps.
- ±24 hours: same time, one day later or earlier.
- +48 or +72 hours: same time, 2 or 3 days later.
Adding more than 24 hours
Split the hours into whole days plus remaining hours (see hours to days):
- Monday 8:00 AM + 50 hours = 2 days + 2 hours → Wednesday 10:00 AM.
- Friday 6:00 PM − 30 hours = 1 day + 6 hours → Thursday 6:00 PM − 6 hours → Thursday 12:00 PM (noon).
Watch out for daylight saving time
When clocks change, elapsed hours and clock hours stop matching. In the US, on the second Sunday in March, the clock jumps from 2:00 AM to 3:00 AM, so 1:00 AM + 2 hours is 4:00 AM, not 3:00 AM. On the first Sunday in November, the clock falls back from 2:00 AM to 1:00 AM, so 12:30 AM + 2 hours is 1:30 AM (the second one). If your calculation spans one of these nights, adjust by an hour. See daylight saving time explained.
Adding hours across time zones
For flights and calls, separate the two steps: add the duration first, then convert the time zone.
Example: a flight leaves New York at 7:00 PM EDT and takes 7 hours. It lands at 2:00 AM EDT, which is 7:00 AM in London (BST, 5 hours ahead). Use the time zone converter for the second step, and read time zones explained for background.
Where this comes up
- Medication schedules: "every 8 hours" starting at 7 AM is 7 AM, 3 PM and 11 PM. Always follow the instructions from your pharmacist or doctor.
- Shift work and timesheets: a shift from 10:00 PM to 6:30 AM is 8 hours 30 minutes.
- Fasting windows: a 16-hour fast after an 8:00 PM dinner ends at 12:00 PM the next day.
- Deliveries and deadlines: "within 48 hours" of 4:15 PM Tuesday means by 4:15 PM Thursday.
Skip the math: the hours from now calculator gives the exact time and date in your own time zone, with daylight saving time handled. Try 8 hours from now or 12 hours from now.
Frequently asked questions
What time was it 8 hours ago?
Subtract 8 from the current hour on the 24-hour clock, adding 24 if the result is negative. At 2:00 PM, 8 hours ago was 6:00 AM.
What time will it be in 12 hours?
The same clock time with AM and PM swapped. 9:20 AM becomes 9:20 PM.
How do I calculate hours between two times?
Convert both to 24-hour time and subtract the start from the end. If the end is earlier, add 24 hours, because the period crosses midnight.