Working out what time it will be in a few hours sounds easy, until the answer crosses noon or midnight. Here is a method that works every time, with no calculator.
Step 1: Switch to the 24-hour clock
The 24-hour clock removes the AM/PM problem. Morning times stay the same (9:00 AM is 09:00). For afternoon and evening times, add 12 (3:00 PM is 15:00). Midnight is 00:00.
Step 2: Add the hours and minutes
Add the hours to the hour, and the minutes to the minutes. If the minutes reach 60 or more, take away 60 and add 1 to the hour.
15:40 + 2 hours 35 minutes → 17:75 → 18:15 (6:15 PM)
Step 3: Subtract 24 for each new day
If the hour is 24 or more, take away 24 and move to the next day. Do it twice for 48 or more.
21:00 + 6 hours = 27:00 → 27 − 24 = 03:00 the next day (3:00 AM)
Worked examples
| Start | Add | Result | Day |
|---|---|---|---|
| 9:00 AM | +5 h | 2:00 PM | the same day |
| 2:00 PM | +8 h | 10:00 PM | the same day |
| 9:00 PM | +6 h | 3:00 AM | the next day |
| 11:00 PM | +12 h | 11:00 AM | the next day |
| 4:00 PM | +18 h | 10:00 AM | the next day |
| 7:00 AM | +30 h | 1:00 PM | the next day |
Shortcuts worth remembering
- +12 hours: same clock time, swap AM and PM.
- +24 hours: same time tomorrow.
- More than 12 hours: count back (24 − hours) on the clock, then move to the next day. +18 hours from 4:00 PM = 6 hours back = 10:00 AM tomorrow.
- Multiples of 24: 48 hours = 2 days, 72 hours = 3 days, 96 hours = 4 days.
Watch out for daylight saving time. If the clocks change during the hours you are counting, the real clock time is one hour different. In the US this happens on the second Sunday in March and the first Sunday in November. Our hours from now calculator adjusts for it automatically.
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