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Duration arithmetic

Time Calculator

Add or subtract two durations with exact whole-second arithmetic, normalized units, and explicit boundaries.

Free · Private · Runs in your browser

Exact duration arithmetic

Add or subtract two durations

Enter whole days, hours, minutes, and seconds. Inputs and results stay in your browser.

First duration
Second duration

Each duration accepts 0–1,000,000 days, 0–23 hours, and 0–59 minutes and seconds. Components must be nonnegative whole numbers.

The normalized duration will appear here after you submit two valid durations.

Formula and calculation method

The calculator represents each duration as an exact whole number of seconds. It uses the fixed relationships 1 minute = 60 seconds, 1 hour = 3,600 seconds, and 1 day = 86,400 seconds.

For duration parts d, h, m, ands, total seconds equal d × 86,400 + h × 3,600 + m × 60 + s. The tool adds those totals or subtracts the second from the first, then decomposes the absolute result back into canonical parts. A negative subtraction keeps one leading minus sign.

Whole-second calculation is exact within the declared input range. Decimal hours are a secondary display rounded once to at most 12 significant digits; the normalized duration and total seconds remain the authoritative calculator outputs.

Inputs, outputs, and boundaries

  • Inputs: an operation plus two durations, each containing whole days, hours, minutes, and seconds.
  • Outputs: a signed normalized duration, exact total seconds, decimal hours, and the seconds equation.
  • Bounds: each duration accepts 0–1,000,000 days, 0–23 hours, 0–59 minutes, and 0–59 seconds.
  • Zero: all-zero durations are valid and produce a zero result.
  • Negative values: input components cannot be negative. Subtraction may produce a negative result, shown with one leading minus sign.
  • Invalid input: blank, signed, decimal, exponent-form, nonnumeric, noncanonical, and out-of-range components are rejected; focus moves to the first invalid field.

Aletheia conventions

“Day” means a fixed duration of exactly 86,400 seconds for this arithmetic tool. It does not mean a local civil day that might be shortened or lengthened by daylight-saving transitions. The calculator uses canonical component ranges instead of silently accepting 90 minutes or 25 hours; normalize those values before entry.

Limitations and disclaimer

This is a duration calculator, not a clock, calendar, timer, or scheduling system. It does not calculate time of day, dates, months, years, time zones, daylight-saving changes, UTC, leap seconds, payroll time, billing rules, or legal deadlines.

Results are arithmetic, not guarantees or professional advice. Use the controlling clock, calendar, contract, regulation, or domain procedure for consequential timing decisions.

Worked examples

Adding 1 hour 30 minutes to 45 minutes converts the inputs to 5,400 seconds and 2,700 seconds. Their 8,100-second sum normalizes to 2 hours 15 minutes.

Subtracting 2 hours 30 minutes 15 seconds from 1 hour gives −5,415 seconds, displayed as −1 hour 30 minutes 15 seconds.

Common questions

Why can’t I enter 90 minutes?

Keeping minutes within 0–59 makes every input unambiguous. Enter 90 minutes as 1 hour 30 minutes.

Does a day always mean 24 hours?

In this duration calculator, yes: one day is exactly 24 hours. A local civil day can behave differently around daylight-saving changes and is outside this tool’s scope.

Why can subtraction be negative?

The calculator preserves operand order. If the second duration is longer than the first, the exact difference is negative instead of silently taking an absolute value.

Use the Date Duration Calculator for calendar dates, or the Unit Conversion Calculator for supported measurement categories other than time.

Version and verification

Calculation version 1.0.0; formulas, boundaries, and deterministic fixtures last verified 2026-08-11. Authority pages were retrieved 2026-08-11. Automated coverage includes exact arithmetic, bounds, negative and zero behavior, privacy, keyboard use, responsive layout, contrast, and Axe. Manual VoiceOver remains waived and is not claimed as passed.