The 24-Hour Clock Method
The most reliable way to add or subtract hours is to work in 24-hour (military) time, where midnight is 00:00 and 11:59 PM is 23:59. This eliminates AM/PM confusion and makes arithmetic straightforward.
- Convert the start time to 24-hour format if needed
- Add or subtract the hours (and minutes) directly
- If the result is ≥ 24:00, subtract 24 to get the next-day time
- If the result is < 00:00, add 24 and note it's the previous day
Adding Hours: Worked Examples
| Start Time | Add | Result | Notes |
|---|---|---|---|
| 9:00 AM | + 8 hours | 5:00 PM (17:00) | Same day, simple |
| 10:30 PM (22:30) | + 3 hours | 1:30 AM (01:30) | Crosses midnight |
| 11:45 PM (23:45) | + 45 min | 12:30 AM (00:30) | Next day |
| 6:00 PM (18:00) | + 12 hours | 6:00 AM (06:00) | Next day |
| 2:15 AM (02:15) | + 9h 30m | 11:45 AM (11:45) | Same day |
Subtracting Hours: Worked Examples
| Start Time | Subtract | Result | Notes |
|---|---|---|---|
| 3:00 PM (15:00) | − 6 hours | 9:00 AM | Same day |
| 1:00 AM (01:00) | − 3 hours | 10:00 PM (22:00) | Previous day |
| 12:15 AM (00:15) | − 45 min | 11:30 PM (23:30) | Previous day |
| 8:00 AM (08:00) | − 10 hours | 10:00 PM (22:00) | Previous day |
Overnight Shift Calculations
If your shift starts at 10:00 PM and ends at 6:30 AM, the duration is: convert both to minutes from midnight, subtract. 10:00 PM = −120 min from midnight (or +1,320 from start of day). 6:30 AM = 390 min from midnight. Duration = (1,440 − 1,320) + 390 = 120 + 390 = 510 minutes = 8 hours 30 minutes.
Simpler shortcut: count from start time to midnight (2 hours), then from midnight to end time (6 hours 30 minutes). Total = 8 hours 30 minutes.
Time Zone Additions
When adding time zones, add or subtract the UTC offset. New York is UTC−5, London is UTC+0, Tokyo is UTC+9. A meeting at 3:00 PM New York time is 8:00 PM London time and 5:00 AM (next day) Tokyo time.
The time calculator below handles all of this — add or subtract hours and minutes from any time, with automatic handling of midnight crossings and results in both 12-hour and 24-hour formats.