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ToolBoxGenie

Time Calculator

Calculators · Added 15 August 2026

Three kinds of time arithmetic that spreadsheets make unnecessarily hard: the gap between two clock times, adding or subtracting durations, and shifting a time forward or back. Overnight spans are handled properly, and every result includes decimal hours — the format payroll and invoicing actually want.

What do you want to do?

24-hour clock, HH:MM or HH:MM:SS.

An earlier time is read as the next day.

How to use the time calculator

  1. 1Choose the mode: time between two clock times, duration arithmetic, or adding a duration to a time.
  2. 2For a span, enter the start and end times on the 24-hour clock. An end time earlier than the start is read as the next day.
  3. 3For duration arithmetic, enter hours, minutes and seconds in the three boxes and pick add or subtract.
  4. 4Press Calculate.
  5. 5Read the decimal hours figure if the answer is going into a timesheet or an invoice.

Examples

A normal working day

Input
09:15 to 17:45
Result
8h 30m · 08:30:00 · 8.5 decimal hours

8.5, not 8.30. Entering 8.30 into a payroll spreadsheet underpays by eighteen minutes.

An overnight shift

Input
22:00 to 06:00
Result
8h · flagged as crossing midnight

Read as the next day rather than as minus sixteen hours, which is what a naive subtraction gives.

Adding durations

Input
2h 45m plus 1h 30m
Result
4h 15m · 4.25 decimal hours

Minutes carry into hours correctly at 60, not 100.

About the time calculator

Why time arithmetic goes wrong

Time is base sixty pretending to be decimal, and almost every error follows from that. Sixty minutes make an hour, sixty seconds make a minute, and a calculator that treats 1.5 hours as one hour fifty minutes is wrong by ten minutes without anything looking unusual.

The decimal conversion is where it bites hardest, because both forms use a decimal point. Eight hours thirty minutes is 8.5 hours in decimal and is often written 8:30 or even 8.30 in notes. Feed the wrong one into a spreadsheet that multiplies by an hourly rate, and the calculation runs happily and pays the wrong amount. Doing this consistently across a team over a year is a meaningful sum.

The second common failure is subtraction across midnight. 06:00 minus 22:00 is minus sixteen hours arithmetically and eight hours in reality. Software that does not special-case this produces negative shifts, and the usual workaround — adding 24 and hoping — breaks the moment a span genuinely exceeds a day.

The formats and where each belongs

There are three ways to write a duration and each has a proper home. Readable form — 8h 30m — is for humans and is unambiguous. Clock form — 08:30:00 — is what most software expects for input and what timestamps use. Decimal form — 8.5 — is for anything that will be multiplied: a rate, an invoice, a payroll run. This calculator shows all three because converting between them by hand is exactly where mistakes enter.

One practical detail worth knowing: spreadsheets store times as fractions of a day, so 8h 30m is 0.354166… internally. Multiplying that by an hourly rate gives a nonsensical answer unless you multiply by 24 first. This is why so many timesheet templates carry a hidden ×24 in a column nobody looks at, and why an apparently correct-looking sheet can be quietly wrong when a formula is copied without it.

For anything that will be audited, the safest habit is to record the clock times themselves rather than a computed duration, and to derive the duration from them. Raw start and end times can be rechecked; a duration typed in by hand cannot be verified against anything once the day has passed.

Frequently asked questions

Why is 8 hours 30 minutes shown as 8.5 and not 8.30?
Because minutes are sixtieths of an hour, not hundredths. Half an hour is 0.5 of an hour, so 8h 30m is 8.5 decimal hours. Writing 8.30 into a timesheet means eight hours and eighteen minutes, and it is one of the most common and most expensive payroll errors there is.
How are overnight shifts handled?
An end time earlier than the start is interpreted as being on the following day, so 22:00 to 06:00 gives eight hours rather than a negative span. The result is flagged so you can see the assumption was made. This is what timesheets need and what a plain subtraction gets wrong.
What is the difference between this and the time converter?
The time converter changes one unit into another — hours into minutes, days into seconds. This calculator does arithmetic on spans and clock times: adding two durations together, finding the gap between two times, shifting a time forward. Different jobs that both involve time.
Can I subtract a break from a shift?
Yes. Find the span from start to finish in the first mode, note the result, then switch to duration arithmetic and subtract the break. A 09:00 to 17:30 shift with a 45-minute break gives 8h 30m minus 45m, which is 7h 45m or 7.75 decimal hours.
Does it handle time zones or daylight saving?
No — it works with clock times as entered, on a single day. For time zone conversions use the time zone converter, which reads the IANA database and knows about daylight saving transitions.

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