Time Calculator
Enter a start time and a duration to add or subtract. Results wrap past midnight and can show 12-hour or 24-hour format.
Calculate
Calculate
Result
The formula
Convert the start clock time to minutes since midnight, add or subtract (hours×60 + minutes), then normalize with modulo 1440. Positive day shifts mean the result falls on a later calendar day; negative shifts mean an earlier day.
Worked example
- Start 09:30 → 570 minutes
- Add 2h 45m = 165 minutes → 735
- 735 → 12:15 (same day)
Result: 09:30 + 2h 45m = 12:15
How time add/subtract works
Clock arithmetic is modular: after 23:59 comes 00:00. This tool tracks how many midnights you cross.
Input formats
Use HH:MM in 24-hour form (09:30, 21:15) or append am/pm (9:30 am). Duration fields are decimal-friendly (e.g. 1.5 hours = 90 minutes when minutes are 0 — prefer whole hours + minutes for clarity).
Overnight wrap
22:00 + 3h = 01:00 next day. 01:00 − 2h = 23:00 previous day. The day-shift badge shows when the result leaves the starting civil day.
What we do not model
No time zones, DST transitions, or seconds. For calendar spans in days, use date difference or days-until.
Interesting facts
1440 minutes/day
A civil day has 24 × 60 = 1440 minutes — the modulus used for wrap-around.
12-hour ambiguity
“12:00 am/pm” conventions vary in speech; we treat 12:00 AM as midnight and 12:00 PM as noon.
Duration ≠ clock
Adding 25 hours is valid: the clock shows +1 day and the leftover hours.
Sibling tools
Date difference counts days between dates; this tool moves a time-of-day by a duration.
Sleep planner
For bedtime + N sleep cycles, use the sleep calculator instead.
Frequently asked questions
Enter 09:30 (or 9:30 am), hours 2, minutes 45, operation Add → 12:15.
Yes. Times wrap at midnight and the result notes +1 day or −1 day when needed.
Enter times with am/pm, or set Display to 12-hour for the result.
Choose Subtract and enter the duration to go backwards from the start time.