Add or Subtract Two Durations
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Time Expression Calculator
Use d, h, m, s for days, hours, minutes, seconds. Separate terms with + or −.
Use d, h, m, s for days, hours, minutes, seconds. Separate terms with + or −.
Of all the units of physical measurement, time is among the most carefully defined. The SI base unit is the second, currently defined since 1967 as exactly 9,192,631,770 oscillations of the cesium-133 atom — a definition precise enough to keep atomic clocks accurate to about one second every 300 million years. All larger time units — minutes, hours, days — are derived from the second by fixed integer multipliers, though the day is not an SI unit because its length is set by Earth's rotation and drifts over geological timescales.
For everyday calculation the relationships are: 60 seconds per minute, 60 minutes (3,600 seconds) per hour, and 24 hours (86,400 seconds) per day. These ratios are why time arithmetic is sometimes called sexagesimal, a base-60 system inherited from Sumerian and Babylonian mathematicians over 4,000 years ago. The base-60 system was retained through antiquity because 60 has an unusually large number of divisors — 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60 — making fractional subdivisions easy without remainders in most practical cases.
ISO 8601, the international standard for date and time representations, defines a compact notation for durations that looks like P1DT2H30M. The leading P stands for "period," and the T separates the date part (years, months, weeks, days) from the time part (hours, minutes, seconds). For example, P1Y2M10DT2H30M means one year, two months, ten days, two hours, and thirty minutes. This format appears in programming APIs, XML schemas, and web standards. The informal expression syntax this calculator accepts — 1h 30m + 2h 45m — is a human-friendly approximation of the same idea, using lowercase unit letters and arithmetic operators instead of the structured ISO form.
| Unit | Seconds | Minutes | Hours |
|---|---|---|---|
| 1 minute | 60 | 1 | 0.0167 |
| 1 hour | 3,600 | 60 | 1 |
| 1 day | 86,400 | 1,440 | 24 |
| 1 week | 604,800 | 10,080 | 168 |
| 1 month (avg) | 2,629,800 | 43,830 | 730.5 |
| 1 year (common) | 31,536,000 | 525,600 | 8,760 |