How to Add and Subtract Time
Step-by-step methods for adding and subtracting hours, minutes, and seconds, with worked examples you can check in the calculator.
Updated
Short answer
Adding time is ordinary addition with one twist: minutes and seconds roll over at 60, not at 10. Carry 60 when adding, and borrow 60 when subtracting.
Once you know when to carry and when to borrow, you can add or subtract any two lengths of time by hand. This guide walks through how to add time step by step, how to subtract it, and what changes when you're working with a clock time instead of a length of time.
If you just need the answer, the timer calculator adds and subtracts hours, minutes, and seconds for you and can show the carrying steps.
Durations vs clock times
Before you start, decide which of these you have:
- A duration is a length of time, like 2 hours 40 minutes. Durations can be added together and can go past 24 hours. Three 9-hour days add up to 27 hours.
- A clock time is a point in the day, like 10:30 PM. You can add a duration to a clock time to find an end time, but you can't add two clock times together. 3:00 PM plus 4:00 PM doesn't mean anything.
Most mistakes come from mixing the two. The rules below cover both.
How to add time
Line the times up in columns (hours, minutes, seconds) and add from the right, the same way you add numbers.
- Add the seconds. If the total is 60 or more, subtract 60 and carry 1 minute.
- Add the minutes, including any carried minute. If the total is 60 or more, subtract 60 and carry 1 hour.
- Add the hours, including any carried hour.
Example: 2 h 40 min 30 s + 1 h 25 min 50 s
- Seconds: 30 + 50 = 80 seconds. 80 − 60 = 20, so write 20 s and carry 1 minute.
- Minutes: 40 + 25 + 1 = 66 minutes. 66 − 60 = 6, so write 6 min and carry 1 hour.
- Hours: 2 + 1 + 1 = 4 hours.
The answer is 4 hours 6 minutes 20 seconds.
Adding a long list
With several times, add each column first and carry at the end. For 45 min + 50 min + 35 min, the minutes add up to 130. 130 minutes is 2 hours (120 minutes) with 10 left over, so the total is 2 h 10 min. Divide by 60: the whole number is hours, and the remainder is minutes.
How to subtract time
Subtracting works right to left too, but instead of carrying you borrow.
- Subtract the seconds. If the top number is smaller, borrow 1 minute from the minutes column and add 60 to the seconds.
- Subtract the minutes. If the top number is smaller, borrow 1 hour and add 60 to the minutes.
- Subtract the hours.
Example: 5 h 15 min − 2 h 40 min
- Minutes: 15 − 40 won't work, so borrow 1 hour. 5 h 15 min becomes 4 h 75 min.
- Minutes: 75 − 40 = 35 minutes.
- Hours: 4 − 2 = 2 hours.
The answer is 2 hours 35 minutes.
Quick check: add your answer back to the number you subtracted. 2 h 35 min + 2 h 40 min = 4 h 75 min = 5 h 15 min, which matches where you started.
Adding time to a clock time
To find when something ends, add the duration to the start time, then deal with AM, PM, and midnight. The easiest way is to switch to 24-hour time first.
Example: 10:30 PM + 7 hours 30 minutes
- 10:30 PM is 22:30 in 24-hour time.
- 22:30 + 7:30 = 29:60, which carries to 30:00.
- Past 24:00 means you've crossed midnight. 30:00 − 24:00 = 6:00.
The answer is 6:00 AM the next day. The What time will it be tab does this for you, from now or from any start time, and can start a countdown to the answer.
Subtracting clock times to find a length of time
To find how long something took, subtract the start time from the end time. If the end is earlier in the day than the start, the span runs past midnight, so add 24 hours to the end time first.
For example, 10:30 PM to 6:45 AM: 6:45 becomes 30:45, and 30:45 − 22:30 = 8:15, so the span is 8 hours 15 minutes. The time duration calculator gives the answer, and you can see elapsed time with the steps shown for the same subtraction.
When the answer is negative
If you subtract a longer duration from a shorter one, the result is negative. 1 h 20 min − 2 h 5 min is −45 minutes. Work it out the other way around (2 h 5 min − 1 h 20 min = 45 min) and put a minus sign in front. The timer calculator marks negative totals so you don't miss them. For this example it shows −0 h 45 min 0 s, which is the same 45 minutes written with a zero in the hours place.
Converting the result to decimal hours
Some forms ask for time as a decimal. Divide the minutes by 60 and add them to the hours: 4 h 6 min is 4 + 6 ÷ 60 = 4.1 hours. Seconds are divided by 3,600. To go the other way, multiply the decimal part by 60: 1.75 hours is 1 hour plus 0.75 × 60 = 45 minutes. The timer calculator shows decimal hours under every total, rounded to two places, and the time unit converter switches any amount between seconds, minutes, hours, and days.
Key points
- Carry at 60 when adding, and borrow 60 when subtracting.
- Durations can go past 24 hours. Clock times roll over at midnight.
- Switch to 24-hour time when a clock time crosses noon or midnight.
- Check a subtraction by adding the answer back.
When you'd rather not do it by hand, the timer calculator adds or subtracts up to 20 times at once and shows the carrying steps.
This guide was written by Cedrick Reese for Timer Calculator. Each worked example here was entered into the timer calculator and came out the same there, so you can rerun any of them yourself and get the answer on this page. The method is plain arithmetic in base 60, so it doesn't change over time.