Time Converter

How to use the time converter

Type a duration, pick the unit it is in, pick the unit you want, and the answer updates as you type. Every unit stores how many seconds it is worth, so any pair converts through that base: milliseconds to minutes, hours to weeks, days to years, and back the other way with the swap button. Eight units are covered — millisecond, second, minute, hour, day, week, month, year.

What this tool measures is elapsed duration: how long something lasts, not what the clock reads. It has nothing to do with time zones, so if you are trying to work out what 3 p.m. in London is in Chicago, this is the wrong tool. It also does not know what today is, which matters once you get to months and years.

Everything from milliseconds up to weeks is exact by definition. A minute is 60 seconds, an hour is 3,600, a day is 86,400, a week is 604,800, and none of those numbers is a rounding of anything. Months and years are different, and the difference is not a technicality. Calendar months run 28, 29, 30 or 31 days, so there is no single number of seconds in a month; a year is 365 days or 366.

The only honest way to put a month or a year on a converter dropdown is to use an average and say so, which is what this tool does. Any result involving months or years is therefore an approximation of a specific calendar span, accurate in aggregate and off by up to a day and a half on a single month.

Use the average when the span is generic and you have no particular dates — a 6-month subscription, a 90-day payment term expressed in months, a device rated for 50,000 hours of life. Use a real calendar whenever the answer depends on which dates you mean, because February and a leap year will not average themselves out over a short span.

For that, the age calculator at /date/age-calculator gives an exact years, months and days breakdown from a birth date. A rough guide: under a year, and where the dates are known, calendar-exact is worth the extra step; longer than that, or with no dates in hand, the average is the better answer.

Where does 365.2425 come from?

The average used here is the Gregorian one: 365.2425 days per year. That number comes straight out of the leap-year rule. Every fourth year is a leap year, except century years, except again those divisible by 400 — so a full 400-year cycle contains 97 leap days rather than 100.

That gives 400 × 365 + 97 = 146,097 days, and 146,097 ÷ 400 = 365.2425 days exactly. Multiply by 86,400 and a year is exactly 31,556,952 seconds. A month is a twelfth of that: 30.436875 days, or exactly 2,629,746 seconds. Both are exact fractions of a real repeating cycle, not a rounded guess, which is why they come out as clean whole numbers of seconds.

The formula, worked line by line

Each unit stores one number: how many seconds it is worth. Converting between any two is a pivot through that base — multiply into seconds, divide out into the target — so a single expression handles every pair, including the two average-based ones.

seconds = value × from.perBase
result = seconds ÷ to.perBase
result = value × from.perBase ÷ to.perBase
1 year = 365.2425 × 86,400 = 31,556,952 s;  1 month = 31,556,952 ÷ 12 = 2,629,746 s

Anything that touches the month or year dropdown inherits the average, and the error is not evenly spread. The average month is 30.436875 days, so it overstates a 28-day February by 2.44 days and understates a 31-day month by 0.56 — an error of about 8.7 percent at the February end and 1.8 percent at the other.

Across a full year the twelve errors nearly cancel, and across the 400-year cycle they cancel exactly by construction, so the average is at its best over long horizons and at its worst over a single short month.

Instant answers for the common conversions

105 conversions get their own page, each with the arithmetic worked out, the clock reading, the same span in every other unit, and what that duration is in practice.

Questions people ask