Convert time

About the units

• Milliseconds (ms) – A millisecond is a thousandth of a second, milli meaning 10⁻³, and a legal unit throughout the EU. Programmers meet it constantly: JavaScript counts time in milliseconds since 1 January 1970 UTC, while Unix and POSIX count seconds from the same instant. The trap follows at once – a timestamp read in the wrong unit is out by a factor of a thousand, and a date column suddenly shows 1970 or a year far ahead. The symbol is ms. mS is the millisiemens, another quantity entirely.
• Seconds (s) – The second is the SI base unit of time, fixed by the General Conference on Weights and Measures. Since 1967 it has been defined through the caesium-133 atom: the second is the time it takes the caesium frequency to oscillate 9,192,631,770 times, restated in 2018 as a defined value for that frequency. It is a legal unit throughout the EU and the basis of every measure of speed, power and frequency. The SI second has nothing to do with the earth’s rotation: leap seconds adjust UTC.
• Minutes (min) – A minute is 60 seconds. It is not an SI unit, but the SI Brochure lists it among the long-standing units of time, and the EU units directive lists it among the legal units. EU type-approval papers use it the other way round: engine speed is written min⁻¹. The symbol is min, not m, which is the metre, and not ′, which is the arcminute. No prefix may be attached to the minute.
• Hours (h) – An hour is 60 minutes, 3,600 seconds. Like the minute it is not an SI unit but is one of the long-standing units of time in the SI Brochure, and the EU units directive treats it as a legal unit. Since combinations of legal units are legal in turn, the kilometre per hour and the kilowatt-hour follow directly: a kilowatt-hour is exactly 3.6 megajoules because the hour is exactly 3,600 seconds. The symbol is h, lower case and without a full stop. At the daylight-saving change a local day has 23 or 25 hours.
• Days (d) – A day is 24 hours, exactly 86,400 seconds – a number that recurs throughout programming. The unit is not an SI unit but is one of the long-standing units of time in the SI Brochure and is legal in the EU. The unit day is exact; the solar day is not, because the earth’s rotation varies. UTC has so far been held within nine tenths of a second of rotation time by leap seconds, and the General Conference on Weights and Measures decided in 2022 that the limit will be raised by 2035 at the latest.
• Weeks (wk) – A week is seven days, 604,800 seconds. It is the one unit of time here that nobody fixes: the week is missing both from the SI Brochure’s list of units of time and from the EU units directive. It still governs planning, pay periods, pregnancy and European working life. Week numbering differs: the international standard starts the week on Monday with week 1 holding the year’s first Thursday, while American practice starts on Sunday.
• Months (mth) – A month here is one twelfth of the mean Gregorian year: 30.436875 days, exactly 2,629,746 seconds. No real month is that long, since calendar months run from 28 to 31 days, so the conversion suits averages rather than dates. The month is neither an SI unit nor a legal unit of measurement in the EU. In UK statutes the Interpretation Act 1978 settles the question: “month” means calendar month, not 30 days and not this average. A period counted in months therefore follows the calendar.
• Years (yr) – A year here is the mean Gregorian year: 365.2425 days, exactly 31,556,952 seconds. The figure follows from the leap-year rule, which gives 97 leap days in 400 years. The year is neither an SI unit nor a legal unit of measurement in the EU, but it is used in contracts, interest and energy consumption – the EU energy label states a fridge’s consumption in kilowatt-hours per year. The trap is that “a year” means several things: the Julian year is 365.25 days, and interest often counts 360 days.