How to Add and Subtract Time: Decimal Hours, Midnight and Daylight Saving
Time arithmetic breaks in four places: base-60 minutes, decimal hours, midnight and the 12 o’clock hour, and daylight-saving days that are not 24 hours long. Here is each, with tested examples.
Time Is Base 60, Not Base 10
Adding times goes wrong because minutes roll over at 60, not 100. 2 hours 45 minutes plus 1 hour 30 minutes is 3 hours 75 minutes, which is 4 hours 15 minutes, not 4.15 of anything. The safe method is to turn everything into minutes (or seconds), do ordinary arithmetic, and convert back. That is exactly what our Time Calculator does, in whole seconds so nothing is rounded.
Hours and Minutes vs Decimal Hours
Payroll and billing systems want decimal hours, and the classic slip is writing 7 hours 45 minutes as 7.45. Divide the minutes by 60 instead: 45 ÷ 60 = 0.75, so it is 7.75 hours. At $20 an hour the slip costs $6.00 on that one shift. Common conversions:
| Minutes | Decimal hours | Minutes | Decimal hours |
|---|---|---|---|
| 6 | 0.10 | 30 | 0.50 |
| 10 | 0.17 | 40 | 0.67 |
| 15 | 0.25 | 45 | 0.75 |
| 20 | 0.33 | 50 | 0.83 |
Clock Arithmetic and Midnight
Adding a duration to a clock time can cross midnight: 9:30 PM plus 4 hours 45 minutes is 2:15 AM the next day, and 1:15 AM minus 26 hours is 11:15 PM two days earlier. For a shift that runs past midnight, subtract the start from the end and add 24 hours if the result is negative: 10:00 PM to 6:30 AM is 8 hours 30 minutes. The Hours Calculator does this for a whole week of shifts.
12-Hour and 24-Hour Time
The 12 o’clock hour causes most conversion errors. 12:00 AM is midnight, the first moment of the day (0000 in 24-hour or military time), and 12:00 PM is noon (1200); 1 PM to 11 PM become 13 to 23 by adding 12. 2400 is sometimes written for the end of a day and means the same moment as 0000 the next day. The Military Time Converter converts either way; we tested it on every one of the 1,440 minutes of the day.
Daylight Saving: Days That Are Not 24 Hours
Where clocks change, two days a year are not 24 hours long, so the time between two clock readings can differ from the time that actually passes. From the time-zone database that phones and computers use, for 2026:
| Place | Short day | Long day |
|---|---|---|
| New York | March 8 (23 hours) | November 1 (25 hours) |
| London | March 29 (23 hours) | October 25 (25 hours) |
| Lord Howe Island, Australia | October 4 (23.5 hours) | April 5 (24.5 hours) |
| Phoenix, Arizona | none | none |
A night shift from 10 PM on March 7 to 6 AM on March 8, 2026 in New York shows 8 hours on the clock but lasts 7, because the clocks jump from 2 AM to 3 AM. The Time Calculator’s “time between” mode reports the real elapsed time and flags the difference. We tested it against the time-zone database in New York, London, Lord Howe Island (whose clocks move by only 30 minutes) and Kolkata, which has no daylight saving.
Counting Days
For spans of whole days, count calendar dates rather than hours, so a daylight-saving change cannot add or remove a day; and decide whether the last day counts. The Date Calculator and our guide on counting days cover that.
How We Checked
All figures come from the tools’ own code: duration sums checked against integer arithmetic, clock arithmetic and military time against Python’s datetime, and elapsed times against Python’s zoneinfo, which reads the same IANA time-zone database.
Add or subtract time
Durations, clock times and the real time between two dates.
Frequently Asked Questions
How do I add hours and minutes?
What is 7 hours 45 minutes in decimal?
Is 12 AM midnight or noon?
Why is my overnight shift an hour short in March?
Which days in 2026 are not 24 hours long?
Published Oct 14, 2026 by Derek Giordano