Quick answer
Add the signed offset to the time since midnight. Each full day crossed changes the day offset. The clock display stays within 00:00 through 23:59.
Step by step
- Convert the starting clock to 24-hour time and count minutes since midnight. Use seconds instead if the offset includes seconds.
- Add a positive offset for later or subtract for earlier. A day contains 1,440 minutes in this calculation.
- If the total reaches 1,440, subtract 1,440 for each day crossed. For a negative total, add 1,440 until it is in range, moving back one day per addition.
Worked examples
Next day
23:45 is 1,425 minutes after midnight. Add 30 to get 1,455, then subtract 1,440. The result is 00:15 the next day, or 12:15 a.m.
Previous day
00:10 − 25 minutes gives −15. Add 1,440 to get 1,425, or 23:45 the previous day (11:45 p.m.).
More than one day
08:00 + 49 hours lands at 09:00 two days later.
What to check
Use this method for a known starting clock and uniform 24-hour days. A real appointment across a daylight-saving or time-zone change also needs the date and time zone.
Related guides
Sources
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