Time Calculator
Add/subtract durations or compute the difference between two times—seconds included. Clear steps and a clean final result.
Your time
Steps to compute the time difference
Start = 2025-08-21 09:00:00— Compose full datetimeEnd = 2025-08-21 17:00:00— Compose full datetimeΔ = End − Start = 28,800 s— Absolute seconds differenceBreakdown = 0y 0mo 0d 8h 0m 0s— Calendar-accurate conversion
Final result
Results interpretation
- Between times shows the total seconds and a readable H:M:S breakdown.
- Add/Subtract shows the final clock time and whether it rolls to previous/next day(s).
- Use optional dates when spans cross midnight and you need calendar-accurate day counts.
How this calculator works
Times are converted to seconds, combined arithmetically, then normalized back into hours, minutes, and seconds. Day rollover is handled by modular arithmetic with an explicit day offset.
Show math & assumptions
- Convert
HH:MM:SSto seconds:t = 3600·H + 60·M + S. - Difference:
|t₂ − t₁|; if no dates andt₂ < t₁with “Next day,” uset₂ + 86400. - Add/Sub:
t' = t ± d, wheredis duration in seconds. - Normalize:
w = ((t' mod 86400) + 86400) mod 86400, day offset= ⌊(t' − w)/86400⌋. - Breakdown:
H = ⌊w/3600⌋,M = ⌊(w mod 3600)/60⌋,S = w mod 60.
FAQ
Do I need to enter dates for a simple time difference?
No. If you leave dates blank, we assume both times are on the same day and can roll to the next day when enabled.
Can I add hours that exceed 24?
Yes. Durations can be very large—day rollover is shown as an offset.
Does the calculator handle leap seconds or time zones?
No—this is a clock-time tool. For time zones or DST-aware scheduling, use a datetime calculator.
Why is there a day offset on my result?
Adding or subtracting durations can cross midnight. The offset tells you whether the result is on a different day.
How precise are the results?
We operate to the second. Inputs accept seconds via the time picker’s “step: 1”.
Can I copy a link with all inputs?
Yes—use “Copy Link with Inputs” to preserve state in the URL.
Use cases & examples
- Shift planning: 08:30 → add 9:15 → 17:45 (same day).
- Countdown timing: Between 22:10 and 01:40 next day → 3:30 with “Next day” enabled.
- Workout/intervals: 00:05:30 × sets → track cumulative time with add mode.
How this calculator works
Times are converted to seconds, combined arithmetically, then normalized back into hours, minutes, and seconds. Day rollover is handled by modular arithmetic with an explicit day offset.
Show math & assumptions
- Convert
HH:MM:SSto seconds:t = 3600·H + 60·M + S. - Difference:
|t₂ − t₁|; if no dates andt₂ < t₁with “Next day,” uset₂ + 86400. - Add/Sub:
t' = t ± d, wheredis duration in seconds. - Normalize:
w = ((t' mod 86400) + 86400) mod 86400, day offset= ⌊(t' − w)/86400⌋. - Breakdown:
H = ⌊w/3600⌋,M = ⌊(w mod 3600)/60⌋,S = w mod 60.
FAQ
Do I need to enter dates for a simple time difference?
No. If you leave dates blank, we assume both times are on the same day and can roll to the next day when enabled.
Can I add hours that exceed 24?
Yes. Durations can be very large—day rollover is shown as an offset.
Does the calculator handle leap seconds or time zones?
No—this is a clock-time tool. For time zones or DST-aware scheduling, use a datetime calculator.
Why is there a day offset on my result?
Adding or subtracting durations can cross midnight. The offset tells you whether the result is on a different day.
How precise are the results?
We operate to the second. Inputs accept seconds via the time picker’s “step: 1”.
Can I copy a link with all inputs?
Yes—use “Copy Link with Inputs” to preserve state in the URL.
Use cases & examples
- Shift planning: 08:30 → add 9:15 → 17:45 (same day).
- Countdown timing: Between 22:10 and 01:40 next day → 3:30 with “Next day” enabled.
- Workout/intervals: 00:05:30 × sets → track cumulative time with add mode.
What this Time Calculator does
Time calculator tools make everyday scheduling and timing math fast. This page covers two core jobs: computing the difference between two times and adding or subtracting a duration to a clock time. You’ll see a clear, step-by-step breakdown and a single final answer that calls out day rollovers.
Find the time between two times
Enter a start and an end time. If you don’t include dates, we treat both on the same day and—when “Next day” is on—roll the end time into tomorrow if it’s earlier than the start. If you do include dates, we compute an exact seconds difference across calendar days and return a readable H:M:S with day counts.
Add or subtract a duration
Provide a base time and a duration in hours, minutes, and seconds. The calculator normalizes the result, so you can enter large hours (e.g., 100 hours) and we’ll return the final clock time plus a positive or negative day offset—useful for long runs, on-call rotations, or production schedules.
Why seconds matter
Events often align to the second—deploy windows, broadcast cues, fitness intervals. The inputs accept seconds (via the time field’s second step), and we show exact seconds in the result to avoid rounding surprises.
Common pitfalls
- Forgetting “Next day,” which can undercount evening-to-morning spans.
- Confusing durations with times—durations aren’t clock times; they’re added/subtracted in seconds.
- Mixing time zones—this tool is time-of-day only; use a date-time calculator for offsets or DST boundaries.
Limitations
We assume civil time with 24-hour days and ignore leap seconds. For internationalized schedules, daylight saving transitions, or locale calendars, use a time-zone-aware scheduling tool.
Practical examples
Example 1: The event starts at 21:55 and ends at 00:25 (next day). With “Next day,” the difference is 2 hours 30 minutes.
Example 2: A maintenance window begins 23:15; add 8:45 → the result is 08:00 next day (+1 day).
Example 3: A rehearsal has 6 blocks of 00:12:00 each; add them to see total run time (01:12:00).