How to Calculate the Duration Between Two Times

A shift runs from 10:15 PM to 6:30 AM. How long is that, to the minute? Subtract the two clock faces directly — the way you'd subtract 10:15 from 6:30 — and you get a negative number, which is obviously wrong; the shift didn't last minus four hours. Finding the duration between two clock times is genuinely simple once both times sit on the same number line first. The mistakes only creep in when the times are written in different formats, or when the span runs past midnight.

This guide covers the exact method: converting to minutes, subtracting, and converting back, plus the specific fix for an overnight span and how to turn the answer into decimal hours for a timesheet. If you just need the number for one pair of times right now, the time duration calculator does it instantly. If you want to be able to do it by hand — on paper, in a spreadsheet formula, or in your head — read on.

Every duration calculation between two clock times comes down to the same three moves, no matter what the specific times are:

• Convert both times to minutes past midnight — hour × 60, plus the minutes.
• Subtract the start from the end.
• If the result is negative or zero, add 1440 (the number of minutes in a day) — that's the midnight crossing. Otherwise, convert the minutes back into hours and minutes by dividing by 60.

That's the whole method. Everything below is really just detail on applying those three steps when the inputs get messier: mismatched formats, or a span that runs past midnight.

Start with the simplest case: both times are already written the same way, and the end time is later in the day than the start. Take a workday that runs from 09:15 to 17:45.

No format conversion and no midnight crossing — this is the case the method is built around. The two sections below just add one extra step on top of it.

Real schedules are rarely this tidy. A roster might list the start time in 12-hour AM/PM and the end time in 24-hour, or a spreadsheet might mix both down a single column. Before you can subtract, both times need to sit on the same 24-hour scale.

Converting 12-hour to 24-hour only touches the hour: 12 AM becomes 00, the hours 1 through 11 AM keep their number, 12 PM stays 12, and 1 through 11 PM each get 12 added. Going the other direction, the full rules and worked examples are in How to Convert 24-Hour Time to 12-Hour Time, or use the 12 to 24 hour converter to do either direction instantly.

Take a shift that starts at 8:15 AM and ends at 17:20. Convert the AM time first: 8:15 AM becomes 08:15, which is 495 minutes. The end time is already 24-hour: 17:20 is 1040 minutes. Subtract: 1040 − 495 = 545 minutes, or 9 hours 5 minutes.

The one case that trips up a plain subtraction is a span that runs past midnight — an overnight shift, a flight that lands the next calendar day, a late film. Subtracting the start from the end gives a negative number, and the fix is exactly what step three of the method says: add 1440 minutes.

If the confusion is really about which one of 12:00 AM or 12:00 PM you're starting from, that's a labelling question, not a duration one — see Is 12:00 AM Midnight or Noon? The Confusion, Settled before you convert.

Once you have the duration in hours and minutes, payroll systems and spreadsheets usually want it as one decimal number instead. The conversion is simple: divide the minutes by 60. The 8h 15m shift above becomes 8 + (15 ÷ 60) = 8.25 decimal hours. For every combination of minutes and the rounding conventions payroll systems actually use, see How to Convert Minutes to Decimal Hours for Payroll.

Enter a start and end time below — 12-hour or 24-hour, it doesn't matter which — and the duration updates as you type, including the midnight crossing.

• Subtracting the clock times directly without converting to minutes first — hour-and-minute subtraction has to borrow across the 60-minute mark, and it's easy to get the borrow wrong by hand.
• Forgetting to convert a 12-hour time before comparing it with a 24-hour one — 8:15 AM and 08:15 only line up once you've actually checked the AM/PM label.
• Getting a negative or zero result and not recognizing it as the midnight-crossing signal from step three.
• Entering the exact same time as both start and end. Two identical clock times shouldn't read as no time at all for shift-length purposes, so this method treats it as a full 24-hour span, using the same add-1440 rule that applies to a negative result.
• Rounding to decimal hours before adding two shifts together, which compounds rounding error. Add the raw minutes first, then convert the total to decimal once.

"Duration between two times" answers one specific question: given a fixed start and a fixed end, exactly how much time separates them, to the minute. A few similar-sounding questions are worth telling apart:

• "How many hours between two times?" usually wants a fast, rounded answer rather than a minute-exact figure. If that's closer to what you need, the shortcut in How Many Hours Between Two Times? A Simple Method gets there faster.
• Elapsed time measures from a fixed starting moment up to right now, rather than between two times you already know. The rules for that are in How to Calculate Elapsed Time.
• If what you're comparing are dates rather than clock times — working days between two dates, say — that's calendar math, not clock math. See How to Calculate Business Days Between Two Dates.
• If you're combining a time with a length of time, rather than finding the gap between two times — adding 2 hours 45 minutes to 6:30 PM, say — that's addition, not duration. See How to Add and Subtract Time (Hours and Minutes).