Time Calculator

About the Time Calculator

Five small calculators for everyday time arithmetic. Add or subtract hours and minutes from a clock time and see whether the answer falls on the next or previous day. Add up a list of durations such as 1:25:30 + 0:48:45. Multiply or divide a duration — four 1 h 15 min sessions, or a shift split three ways. Convert between hours:minutes:seconds and decimal hours or minutes for timesheets and payroll. And switch between 12-hour, 24-hour and military time, with a reference chart.

Every answer is shown in several notations at once — 12-hour, 24-hour, words, decimal hours and the ISO 8601 duration format — and nothing you type leaves your browser.

Common uses

• Working out when a train, flight, exam or shift ends from its start time and length.
• Adding up the lengths of video clips, laps, lessons or billable sessions.
• Turning hours and minutes into decimal hours for a timesheet or invoice, and back.
• Reading 24-hour and military times on tickets, rosters and schedules.

How durations are written

A duration can be typed in several ways, and all of them mean the same thing:

• 1:30 — hours and minutes (not minutes and seconds); 1:30:15 adds seconds. Hours can go past 24, as in 125:00.
• 1h 30m, 1 hour 30 minutes, 90 min, 5400 s, 2d 4h.
• 1.5 h or 0.75 hours — decimal hours.
• PT1H30M — the ISO 8601 form that the calculator also shows with every result.

A bare number such as 90 is not accepted, because it could mean minutes or hours; add a unit or use the h:mm form.

Decimal hours

Timesheets and payroll often record time as decimal hours. To convert hours and minutes to decimal hours, divide the minutes by 60: 7 h 45 min = 7 + 45 ÷ 60 = 7.75 hours. To go back, multiply the decimal part by 60: 0.2 hours = 12 minutes, so 8.2 hours = 8 h 12 min.

Some conversions never end — 20 minutes is 0.3333… hours — so the result is rounded to four decimal places and marked with ≈. Six minutes is exactly 0.1 hours — the step used when time is billed in tenths of an hour.

12-hour, 24-hour and military time

The 24-hour clock counts hours from 00 to 23, so 1 PM is 13:00 and 11:59 PM is 23:59. To convert a PM time to 24-hour time add 12 to the hour (except 12 PM, which stays 12); to convert back, subtract 12 from hours 13 to 23 and write PM.

Midnight is the tricky part: 12:00 AM is 00:00, the start of the day, and 12:30 AM is 00:30. 12:00 PM is noon. The time 24:00 also exists in 24-hour notation, and ISO 8601 allows it again since its 2022 amendment (ISO 8601-1:2019/Amd 1:2022): it means the end of a day, the same moment as 00:00 of the next day.

Military time is the 24-hour clock written as four digits without a colon — 0900, 1345, 2140 — and often followed by the word “hours”.

Limitations

• Clock arithmetic uses a plain 24-hour clock; it does not know time zones or daylight-saving changes. To convert between places, use the time zone converter.
• Durations are capped at 100,000 hours and calculated to the millisecond; decimal results are rounded to four places. ISO 8601 durations are read in weeks, days, hours, minutes and seconds only, because years and months have no fixed length.
• For the time between two clock times or dates, with breaks and several rows, use the time duration calculator.

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