How to Calculate Elapsed Time Between Two Clock Times
Elapsed time is just the difference between a start time and an end time, but converting hours and minutes correctly trips people up because clocks run on base 60, not base 100. Our time calculator does the conversion instantly, but the manual steps below are worth knowing for quick mental math.
Convert to 24 hour time first
Before subtracting, convert both times to 24 hour format so AM and PM cannot cause confusion. Add 12 to any PM hour other than 12 itself, and treat 12 AM as 00. So 9:15 PM becomes 21:15, and 12:30 AM becomes 00:30.
Subtract hours and minutes separately
Subtract the start hour and minute from the end hour and minute independently. If the end minutes are smaller than the start minutes, borrow 60 minutes from the hours column, the same way you would borrow 10 when subtracting in base 10 arithmetic.
Worked example
To find the elapsed time from 9:40 AM to 1:15 PM: convert to 24 hour time, giving 09:40 and 13:15. The minutes, 15 minus 40, need borrowing, so it becomes 75 minus 40 equals 35 minutes, and the hour count drops from 13 to 12. Then 12 minus 9 equals 3 hours. The elapsed time is 3 hours and 35 minutes.
When the time span crosses midnight
If the end time is earlier in the clock than the start time, such as starting at 10:00 PM and ending at 6:00 AM, add 24 hours to the end time before subtracting. 6:00 AM becomes 30:00 in this calculation, so 30:00 minus 22:00 gives 8 hours elapsed, correctly accounting for the overnight span.
Converting minutes to decimal hours for payroll
Timesheets and hourly pay usually need elapsed time expressed as a decimal rather than hours and minutes, since 3 hours 35 minutes is not something payroll software can multiply by an hourly rate directly. Divide the minutes by 60 to get the decimal portion: 35 minutes is 35 divided by 60, or about 0.58. So 3 hours 35 minutes becomes 3.58 hours for pay calculation purposes. This step is where a lot of manual timesheets go wrong, because it is tempting to just write .35 instead of .58, treating minutes like they are already out of 100. Once the hours are right, US rules usually pay time past 40 in a week at 1.5 times the rate; see how overtime pay is calculated.
Clock time is not decimal math
Elapsed time calculations show up constantly: timesheets, cooking, workouts, travel itineraries, and billing by the hour. The classic mistake is treating clock times like decimal numbers. From 9:45 to 10:15 is 30 minutes, but typing 10.15 minus 9.45 into a calculator gives 0.70, which reads like 70 minutes. Clock minutes run to 60, not 100, so subtract hours and minutes separately, or convert both times to minutes first.
Skip the manual math
For timesheets or any calculation where accuracy matters, the time calculator handles the borrowing, decimal conversion, and midnight crossover automatically, so there is no risk of an off by an hour mistake from a skipped carry, and no need to remember the minutes to decimal conversion by hand.
Frequently asked questions
How do you calculate elapsed time between two times?
Convert both times to 24 hour format, then subtract the start time from the end time, borrowing 60 minutes from the hours column when the end minutes are smaller than the start minutes.
How do you calculate elapsed time that crosses midnight?
Add 24 hours to the end time before subtracting, so a time like 6:00 AM becomes 30:00 when the start time was the previous evening.
What is the difference between elapsed time and duration?
They mean the same thing in most everyday use: the amount of time that passed between a start point and an end point, expressed in hours and minutes.