Time Duration Calculator

Features

• Exact duration between two clock times
• Overnight option for shifts that cross midnight
• Subtract an unpaid break in minutes
• Shows hours and minutes, decimal hours and total minutes
• Runs entirely in your browser

How to use it

1. Enter the start time and end time.
2. Tick overnight if the end time is on the next day.
3. Optionally subtract a break in minutes.
4. Read the duration in whichever format you need.

Why time subtraction trips people up

Subtracting times feels like it should be as easy as subtracting numbers, but clock time is base 60, not base 10, which is exactly where mistakes creep in. From 9:45 to 10:30 is not 0.85 of an hour just because 30 minus 45 looks negative; it is 45 minutes, because you borrow a full 60-minute hour. The reliable method is to convert both times to minutes since midnight, subtract, and then convert back. 10:30 is 630 minutes, 9:45 is 585 minutes, the difference is 45 minutes.

Overnight shifts add the second trap. If someone clocks in at 22:00 and out at 06:00, a plain subtraction gives a nonsensical negative sixteen hours. The fix is to recognise the end time falls on the next day and add 24 hours' worth of minutes before subtracting, giving the correct eight hours. That is what the overnight option does for you.

Decimal hours matter for anyone billing or being paid by time, because payroll and invoices usually work in decimals rather than hours and minutes. Thirty minutes is 0.5 hours, fifteen is 0.25, and twenty is 0.333, so eight hours and thirty minutes is 8.5, not 8.30. Showing the duration in hours-and-minutes, decimal and total-minutes at once means you never have to convert by hand for a timesheet.