How to Add and Subtract Time in Hours, Minutes and Seconds

Why Time Math Feels Different

Ordinary arithmetic carries at 10. Time carries at 60: 60 seconds make a minute and 60 minutes make an hour, so an hour is 3,600 seconds. That base-60 system goes back to Babylonian astronomy, and it is why 1:50 plus 0:20 is 2:10, not 1:70. The second itself is defined precisely: NIST explains that the SI second is the duration of 9,192,631,770 cycles of the radiation from the cesium-133 atom, and every larger unit of clock time is built from it.

The safest method for any time sum is to convert everything to one unit, do the math, then convert back. Seconds work for stopwatch times; minutes are enough for shifts and schedules.

Adding Time Step by Step

• Add each column separately: seconds with seconds, minutes with minutes, hours with hours.
• Carry from the right: every 60 seconds becomes 1 minute, then every 60 minutes becomes 1 hour.
• Do not wrap at 24 for durations. 20 hours plus 10 hours is 30 hours of elapsed time, even though a clock would read 6:00.

Subtracting Time and Borrowing

Subtraction borrows 60 instead of 10. To work out 5:10 minus 2:45, you cannot take 45 minutes from 10, so borrow one hour: 5:10 becomes 4:70. Then 70 − 45 = 25 minutes and 4 − 2 = 2 hours, giving 2:25. If the second time is larger than the first, the result is negative: 1:00:00 minus 2:30:00 is −1:30:00.

Converting to Decimal Hours

Payroll systems, invoices and spreadsheets usually want decimal hours. Divide minutes by 60 and seconds by 3,600.

Minutes	Decimal hours
6	0.1
15	0.25
20	0.333
30	0.5
45	0.75

A common mistake is reading 1.30 hours as 1 hour 30 minutes. It is actually 1 hour 18 minutes, because 0.30 × 60 = 18.

Time Between Two Clock Times

To find how long passes between two clock times, convert both to a 24-hour clock and subtract. 8:45 AM to 5:15 PM is 08:45 to 17:15, which is 8 hours 30 minutes. If the end time is earlier than the start, the span crosses midnight, so add 24 hours: 10:00 PM to 6:00 AM is 06:00 − 22:00 + 24:00 = 8 hours.

Daylight saving time is the one exception to the simple rule. In the United States it begins at 2:00 AM local time on the second Sunday in March and ends at 2:00 AM on the first Sunday in November, under the Energy Policy Act of 2005. In 2026 that is March 8 and November 1. A shift from 10:00 PM to 6:00 AM that spans the March change lasts only 7 real hours, and the same shift across the November change lasts 9.

Tips for Getting It Right

• Pick one unit (seconds or minutes) before you add a long list of times.
• Label durations and clock times differently. A duration can exceed 24 hours; a clock time cannot.
• Check the DST dates for overnight spans in March and November.
• Round only at the end when converting to decimal hours.

Frequently Asked Questions

How do you add hours and minutes?

Add hours, minutes and seconds in separate columns, then carry every 60 seconds into a minute and every 60 minutes into an hour. 2:45:30 plus 1:30:45 is 4:16:15.

How do you subtract time?

Subtract each column, borrowing 60 instead of 10 when needed. 5:10 minus 2:45 becomes 4:70 minus 2:45, which is 2:25.

How do I convert minutes to decimal hours?

Divide by 60. 15 minutes is 0.25 hours, 30 minutes is 0.5 hours and 45 minutes is 0.75 hours.

How do I calculate time that crosses midnight?

Subtract the start from the end on a 24-hour clock and add 24 hours if the result is negative. 10:00 PM to 6:00 AM is 8 hours.

Sources

• NIST Time and Frequency Division, "Time and Frequency from A to Z: Second" (nist.gov), definition of the SI second.
• NIST, "Daylight Saving Time" (nist.gov/pml/time-and-frequency-division/popular-links/daylight-saving-time-dst), U.S. start and end rules.
• Energy Policy Act of 2005, Section 110 (15 U.S.C. 260a), daylight saving time period.

This guide is for education. For tax, payroll or legal decisions, check the rules that apply to you.