Time Calculator
Add / subtract ยท Duration ยท Convert ยท Overnight
โฑ๏ธ Enter times to add, subtract, measure duration, or convert units.
Time Math Formulas
Time arithmetic is easiest in one base unit. Convert everything to seconds, add or subtract, then expand back to days, hours, minutes, and seconds.
Unit relationships
Add or subtract
Duration between clocks
Modes & FAQ
| Mode | Use when |
|---|---|
| Add / Subtract | Combine two durations |
| Duration | Elapsed between two clocks |
| From Clock | Start time ยฑ duration โ new clock |
| Convert | Express one value in all units |
How to Use This Time Calculator
Pick a mode. Add / Subtract combines two durations built from days, hours, minutes, and seconds. Duration measures elapsed time between two clock times, with optional dates and a break in minutes. From Clock applies a duration to a starting clock. Convert turns one quantity into every common unit.
Results show a normalized D / H / M / S string, decimal hours, totals, an HH:MM:SS view, a chart of the absolute breakdown, and step by step notes.
What time units mean
The SI base unit of time is the second. Minutes and hours are conventional groupings of 60. A civil day here is 24 hours (86400 seconds). This calculator does not adjust for leap seconds.
How to add time manually
1. Write both durations. 2. Add seconds; if โฅ 60, carry minutes. 3. Add minutes with carry to hours. 4. Add hours with carry to days (24 hour days). Example: 1h 50m + 0h 20m โ 2h 10m.
How to subtract time manually
Borrow when needed: 1 minute = 60 seconds, 1 hour = 60 minutes, 1 day = 24 hours. Example: 3h 10m โ 1h 40m โ borrow โ 2h 70m โ 1h 40m = 1h 30m.
Overnight and multi day spans
A shift from 22:00 to 06:00 is 8 hours if you allow overnight. Without overnight, end before start would look negative. When you set both dates, the calculator uses the full date time difference instead of the 24 hour wrap rule.
Decimal hours for payroll
8 hours 15 minutes = 8.25 decimal hours because 15รท60 = 0.25. Many payroll systems store time this way. Total minutes รท 60 gives the same decimal hours value.
Breaks
In Duration mode, enter unpaid break minutes to subtract from the gross span. Example: 09:00 to 17:30 is 8h 30m; a 30 minute break leaves 8h 00m net.
Common mistakes
- Adding clock faces as if they were base 10 (13:70 is invalid)
- Forgetting overnight on night shifts
- Mixing decimal hours with HH:MM without converting
- Treating 100 minutes as 1 hour without dividing by 60
- Ignoring seconds when precision matters
Practical scenarios
Editors sum clip lengths. Runners add split times. Support teams measure ticket handle time. Studios schedule sessions that start at a clock and run a fixed duration. Students convert exam minutes into decimal hours for study logs.
Use the Date Calculator when the question is about calendar days rather than clock duration. Use the Age Calculator when the span starts at a birth date. For repeating weekly schedules, convert each work block with Duration mode, then sum the blocks in Add / Subtract mode.
Sexagesimal grouping
Minutes and hours use base 60 because 60 divides evenly by many small integers. That is why converting 90 minutes to hours is 1.5, not 1.90. Always divide minutes by 60 when moving to hours, and multiply hours by 60 when moving to minutes.
Summing many short clips
Video and audio editors often add many short durations. Convert each clip to seconds, sum once, then normalize. Example: 0:45 + 1:20 + 0:55 = 45 + 80 + 55 = 180 seconds = 3 minutes exactly.
Negative durations
Subtract mode can produce a negative result when B is longer than A. The sign shows in the hero and metrics. Absolute totals still appear so you can see the magnitude without losing the direction of the difference.
HH:MM:SS display
The HH:MM:SS metric folds days into hours. A result of 1 day 2 hours shows as 26:00:00 in that field while the Days metric still shows 1. Use D / H / M / S when you want days separated, and HH:MM:SS when you want a single clock style total.
When this calculator is useful
Use it to add or subtract durations, find elapsed time between two times, project an end clock from a start plus duration, convert units, and export decimal hours for timesheets.
Custom weekends note
This tool measures clock time, not business days. For Monday to Friday day counts, use the Date Calculator business day mode instead.
Minutes and billing rounds
Some workplaces round to the next 6 or 15 minutes. This calculator shows exact elapsed time first. Apply your rounding rule after you read total minutes or decimal hours so the raw span stays transparent.
When this calculator may not be enough
It does not apply time zone offsets or daylight saving transitions. Crossing a DST change can make civil clock spans differ from 3600 second hours. For zone math, use a dedicated time zone tool.
How to interpret your result
The hero shows the primary normalized duration or clock result. Chips list decimal hours and totals. Metrics break out each unit. Day offset appears in From Clock mode when the result crosses midnight.
Worked duration example
Start 09:15, end 17:45 same day. Minutes from midnight: 555 and 1065. Difference 510 minutes = 8 hours 30 minutes = 8.5 decimal hours = 30600 seconds.
Worked from clock example
Start 23:00, add 2 hours. Seconds from midnight 82800 + 7200 = 90000. 90000 โ 86400 = 3600 โ 01:00 next day (day offset +1).
Limitations
Inputs assume non negative field values that are converted to a signed total only by subtraction mode. Leap seconds are ignored. Results are for planning and education, not certified timekeeping.
Sources & References
- NIST: SI Units โ Time (second): nist.gov/pml/owm/si-units-time
- BIPM: SI Brochure โ Unit of time (second): bipm.org/en/si-base-units/second