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Time Calculator

Add and subtract durations, evaluate supported time expressions or shift a civil date and time by an entered duration.

Your inputs

HH:MM:SS; hours may exceed 24.

Calculations run in your browser. No account needed.

EXAMPLE RESULTCalculated locally
Duration
04:15:00
Decimal hours
4.25
Seconds
15,300
Clear inputs. Transparent methods.
My workspace

Calculate with your inputs to save a result or compare scenarios.

How it works

A useful distinction

Carry minutes before comparing totals

Adding 1 hour 50 minutes and 20 minutes produces 2 hours 10 minutes. The extra hour comes from carrying 60 minutes. Keep duration notation consistent when combining task estimates, recordings or split-session totals.

Adding durations and changing a date-time

Duration arithmetic combines lengths of time, while adding time to a date produces a new calendar value. Choose the matching mode before entering the expression. Keep the hour, minute, and second formats consistent, especially when a duration is longer than a day. A decimal fraction of an hour is different from the digits after a colon in a clock-style entry.

Duration arithmetic: formula and assumptions

Convert durations to seconds, perform the operation, and convert back.

Duration arithmetic has no calendar, timezone, or daylight-saving assumption.

Add time to a date: formula and assumptions

Convert the duration to seconds and add it to the civil date/time.

Negative values subtract time. This is timezone-independent civil arithmetic.

Duration expression: formula and assumptions

Total seconds = days×86,400 + hours×3,600 + minutes×60 + seconds.

Expressions are parsed as time quantities, not evaluated as code.

Worked example

Using the Duration arithmetic example:

Inputs
First duration
2:30:00
Operation
Add
Second duration
1:45:00
Method

Convert durations to seconds, perform the operation, and convert back.

Result

Example result

Duration: 04:15:00

Decimal hours: 4.25

Seconds: 15,300

These are illustrative inputs. Change the values to explore a different scenario.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Is a duration of 36 hours valid even though a clock stops at 23?

Yes. Durations measure elapsed quantities rather than a time of day, so they can exceed 24 hours.

Does adding a duration apply daylight-saving rules?

No. Duration arithmetic has no timezone, and date-time addition is civil arithmetic rather than a conversion between zoned instants.

Prepared by Finflo. See our methodology and editorial policy.