Time calculator
Add or subtract a precise duration from a clock time, or measure the elapsed time between two clock readings.
Choose whether to move a clock reading or compare two readings.
Enter a clock time including seconds when they matter.
The day change is shown separately when the duration crosses midnight.
Included
- Adding or subtracting hours, minutes and seconds from a 24-hour clock time
- Elapsed time between two clock readings on the same day
- An explicit next-day option for overnight spans
- 12-hour or 24-hour result display
- Day rollover and signed earlier-time results
Not included
- Calendar dates, weekdays and business-day rules
- Time zones, daylight-saving changes and UTC offsets
- Work breaks, payroll rounding and timesheet rules
- Stopwatch precision below one second
What this means
Clock arithmetic becomes reliable when every time is first converted to seconds after midnight. A duration can then be added or subtracted as one exact number before the result is wrapped back onto a 24-hour clock.
Elapsed mode keeps the day choice visible. Same-day mode reports the signed difference end − start, so an earlier end time produces a negative result. Next-day mode places the end on the following day and returns a forward overnight duration.
The 12-hour or 24-hour choice changes only how clock readings are shown. It does not change the calculation or infer a locale, date or time zone.
Formula & worked example
clock seconds = hours × 3,600 + minutes × 60 + seconds shifted time = start seconds ± duration seconds same-day elapsed = end seconds − start seconds next-day elapsed = end seconds + 86,400 − start seconds
Add 2 hours, 30 minutes and 45 seconds to 23:45:30
- Start
- 23:45:30
- Duration
- 02:30:45
- Unwrapped sum
- 26:16:15
The result is 02:16:15 on the next day.