How to Calculate Time Between Two Times

The arithmetic is easy and the edge cases are where people go wrong. Once you convert both times to minutes after midnight, every case including midnight becomes a single subtraction.

The Basic Method

Convert each time to minutes after midnight by multiplying the hour by 60 and adding the minute. Start at 9:00 is 540 minutes. End at 17:30 is 1,050. The duration is the difference between them.

Handling Midnight

If the end time is earlier than the start, add 1,440 minutes, which is one whole day. A 22:00 to 06:00 shift is 1,440 plus 360 minus 1,320, which is 480 minutes or 8 hours.

The Identical Times Case

A start and end at the same clock time gives zero, which is correct for a duration measurement. If you are counting days rather than measuring a span, an identical time means one full day rather than nothing.

Converting the Result Back

Divide the total minutes by 60 to get decimal hours, and take the remainder for leftover minutes. 510 minutes is 8.5 decimal hours, or 8 hours 30 minutes in clock notation.

The 24 Hour Ceiling

Two clock readings can express at most 1,439 minutes, just under a day. Any longer duration needs days recorded as a separate field rather than folded into the clock values.

Counting Days Versus Measuring a Span

These are different questions. Counting how many days are touched by an event is an inclusive count, so Monday to Wednesday is three days. Measuring how long something takes from Monday 9:00 to Wednesday 9:00 is 48 hours. Mixing the two is a common scheduling error.