How the time calculator works
The calculator converts every value to seconds, does the arithmetic, then converts back. There are 60 seconds in a minute and 3,600 in an hour, so carrying works like base 60 instead of base 10.
Result = A ± B, shown as h:mm:ss
Decimal hours = seconds ÷ 3,600
Time between = end − start (+24 hours if end is earlier)
If the end clock time is earlier than the start, the span is assumed to cross midnight, so 22:00 to 06:00 is 8 hours. Results can exceed 24 hours, and a subtraction that goes below zero is shown with a minus sign.
Worked example
Adding 2:45:30 and 1:30:45: seconds 30 + 45 = 75, which is 1 minute 15 seconds. Minutes 45 + 30 + 1 = 76, which is 1 hour 16 minutes. Hours 2 + 1 + 1 = 4. The total is 4:16:15, or 4.2708 decimal hours. From 8:45 AM to 5:15 PM is 8:30:00, or 8.5 hours.
Frequently asked questions
How do I add hours and minutes?
Add the seconds, minutes and hours separately, then carry every 60 seconds into a minute and every 60 minutes into an hour. 1:45 plus 0:30 is 2:15.
How do I convert minutes to decimal hours?
Divide the minutes by 60. 30 minutes is 0.5 hours, 15 minutes is 0.25 hours and 20 minutes is about 0.333 hours.
How do I calculate time between two times that cross midnight?
Subtract start from end and add 24 hours if the result is negative. 10:00 PM to 6:00 AM is 8 hours.
Can the result be more than 24 hours?
Yes. Durations are not clock times, so 20 hours plus 10 hours is shown as 30:00:00.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.