Time Calculator
Add and subtract time values
About This Calculator
Time arithmetic lets you add and subtract hours, minutes, and seconds — handling the 60-unit rollover of the sexagesimal (base-60) time system. It is useful for calculating total time worked, elapsed time, scheduling, comparing recorded times, and summing durations across multiple events.
Formula
Add times: sum seconds, convert: 60 sec = 1 min, 60 min = 1 hr
Subtract times: if negative, borrow from the next unit
Total hours from h:mm:ss = h + m/60 + s/3600
Duration between times: end − start (add 24 hr if end < start, i.e. crosses midnight)
Example Calculation
Add 2:45:30 + 1:30:45
- Seconds: 30 + 45 = 75 → 1 min 15 sec
- Minutes: 45 + 30 + 1 carry = 76 → 1 hr 16 min
- Hours: 2 + 1 + 1 carry = 4 hr
2:45:30 + 1:30:45 = 4:16:15
Time Addition Reference
| A | B | A + B | A − B |
|---|---|---|---|
| 1:30:00 | 0:45:00 | 2:15:00 | 0:45:00 |
| 2:15:30 | 1:45:45 | 4:01:15 | 0:29:45 |
| 8:00:00 | 4:30:00 | 12:30:00 | 3:30:00 |
| 23:45:00 | 0:30:00 | 24:15:00 | 23:15:00 |
Frequently Asked Questions
How does time arithmetic cross midnight?
Adding times can exceed 24 hours (e.g., 23:00 + 2:00 = 25:00 or 1:00 the next day). Duration calculators typically show total time regardless of day boundaries (showing 25:00:00 rather than wrapping to 1:00:00 AM).
How do I convert decimal hours to hours and minutes?
Take the decimal part and multiply by 60. For example, 2.75 hours = 2 hours + 0.75×60 = 2 hours 45 minutes. Payroll systems sometimes use decimal hours (8.5 hours) while others use HH:MM format (8:30).
What is the 24-hour (military) time format?
The 24-hour format runs from 00:00 (midnight) to 23:59, eliminating AM/PM ambiguity. 13:00 = 1:00 PM, 00:00 = midnight, 12:00 = noon. It is standard in aviation, the military, healthcare, and most non-English-speaking countries.
How do I calculate hours worked between two clock times?
Subtract start time from end time. If you clocked in at 9:15 AM and out at 5:45 PM: 17:45 − 9:15 = 8:30 (8 hours 30 minutes). For overnight shifts, add 24:00:00 to the end time before subtracting.