Time calculator

Why "one month" is not thirty days

Most calculators answer this by dividing seconds, and most are wrong at the edges. A month is 28, 29, 30 or 31 days depending on which one you are standing in. Divide by an average and the answer drifts.

This one counts calendar months instead. One month after 31 Januaryis 28 February — not 3 March. That is what people mean, what contracts mean, and what every serious date library does. In a leap year the same sum lands on 29 February, and the calculator knows which years those are: divisible by four, except centuries, except those divisible by four hundred. 2100 is not a leap year; 2000 was.

Decimal hours

Payroll and timesheets almost never use hours and minutes. They use decimal hours: 7h 30m is 7.5, and 7h 20m is 7.33, not 7.20. That conversion is where most manual timesheet errors come from, so every result here gives both readings side by side.

Shifts that cross midnight

A shift from 22:00 to 06:00 is eight hours, not minus sixteen. The work hours mode detects when the end time is earlier than the start and carries into the next day rather than reporting a negative. Breaks are subtracted from the total, and a break longer than the shift gives zero rather than a negative number.

Approximate units

Converting between seconds, minutes, hours, days and weeks is exact. Converting to months or years is not, because neither has a fixed length — the conversion uses an average month of 30.44 days and a year of 365.2425. Those two are marked as approximate wherever they appear. If you need an exact answer, use the first mode with real dates, which counts the actual calendar.

Business days

Working days are counted as Monday to Friday. Public holidays are not deducted, because they differ by country and often by region — a figure that looked authoritative but was wrong for your location would be worse than none.

Time zones

The date fields use your device's local time and the arithmetic runs on the underlying instants. Across a daylight-saving change the total hours reflect real elapsed time rather than the clock difference — usually what you want, and occasionally surprising.