Add a duration to a start time, or find the duration between two clock times.

How to Use the Hours Calculator

There's a mode switch at the top: adding builds a new clock time from a start point and a duration, while the duration mode goes the other way, comparing two clock times directly. Pick whichever matches the question actually being asked.

Two different questions live under "hours calculator": what time do I end up at if I add 7 hours 30 minutes to 9:15, and how many hours actually passed between two clock times. This tool handles both, switching the math depending on which one you're asking.

Adding:  new time = (start time + duration) mod 24h
Between: duration = end time − start time

The "between" mode needs one more decision from you: does the shift cross midnight? 11 PM to 3 AM is either a nonsensical negative four hours or a perfectly normal four-hour overnight shift, depending entirely on whether you flag it — the checkbox exists because the math genuinely can't tell which one you mean on its own.