Adding time to a clock reading, or taking it away
Shift handovers, dosing intervals and video timecodes all raise the same question: what clock time is a given number of hours and minutes from here? Doing it by hand goes wrong at the 60-minute carry and at midnight. This calculator converts the starting time and the amount to add into seconds, applies them in one step, and converts back, so no intermediate carry can drift.
Reading the midnight rollover
When the answer passes 24:00 or drops below 00:00, the clock face alone hides part of the answer. The whole-day quotient is therefore reported separately as a rollover: +1 day or +2 days when adding, -1 day when subtracting. The result is also printed in 12-hour form so AM and PM are visible at a glance.
The math is only the 60-second, 60-minute and 24-hour conversions, so no reference data is needed, and as of October 2026 no external table or official recommended value is used. Because the tool works on clock times inside one day, adding or subtracting calendar dates is not supported, and neither is adding two durations together. Up to 9,999 hours can be entered.
Frequently asked questions
The rollover line tells you. Adding 5 hours to 22:00 gives 03:00 with a rollover of +1 day, and adding 39 hours to 09:00 gives 00:00 with +2 days.
Yes. Subtracting 5 hours from 02:00 gives 21:00 with a rollover of -1 day. The clock time is always normalized into the 00:00:00 to 23:59:59 range.
This tool works inside a single day, so read the rollover line and adjust the date yourself. Adding or subtracting calendar dates with a year, month and day is not supported.