Fixed units and averaged units
The small end of the scale is exact. A millisecond is a thousandth of a second, a minute is 60 seconds, an hour is 3,600 seconds, a day is 86,400 seconds and a week is 604,800 seconds. Nothing about these depends on the calendar, so converting between them is plain arithmetic. That is why computers count time in seconds or milliseconds since an epoch and only translate to calendar dates at the last moment.
Months and years are different, because real months run 28 to 31 days and years run 365 or 366. To give a usable number this converter uses averages: the month is 30.44 days, which is 365.2425 divided by 12 under the Gregorian rule, and the year is 365.25 days, the Julian average that includes one leap day every four years. A year is therefore 31,557,600 seconds here, and a month is 2,630,016. Those averages are right for rates and estimates, but they are wrong for a specific date range, so use a date calculator when you need the exact number of days between two calendar dates.
Worked examples
Simple ones first: 90 minutes is 1.5 hours; 1 day is 1,440 minutes; a 40-hour work week is 144,000 seconds; and there are 8,760 hours in a 365-day year, or 8,766 using the 365.25-day average. A 45-minute meeting held every weekday for a year is 195 hours, nearly five full work weeks.
A million seconds is 11 days 13 hours 46 minutes 40 seconds; a billion seconds is 31.7 years, which is why people celebrate their billion-second birthday around age 31 years 8 months. A 2.5-hour movie is 9,000 seconds. A video encoded at 30 frames per second gives each frame 33.33 ms. Human reaction time is about 250 ms, a blink is 100 to 400 ms, and a 60 Hz display refreshes every 16.67 ms, which is the budget a game has to draw a frame.
Where the averages go wrong
Payroll is the classic trap. Paying someone monthly means twelve payments a year; paying biweekly means 26 payments, and every eleven years or so 27, because 26 times 14 days is 364, one day short of a year. Subscription billing hits the same issue: a 30-day billing cycle is not a month, and over a year it produces 12.17 cycles. Leases and interest calculations sometimes use a 360-day year of twelve 30-day months, a banking convention that makes daily interest tidy but does not match the calendar.
The real calendar also drifts. A Gregorian year averages 365.2425 days because century years are leap years only when divisible by 400, so 2000 was a leap year and 1900 was not. Leap seconds have occasionally been added to keep clocks aligned with the Earth, though the practice is being phased out by 2035. For durations shorter than a month, the numbers on this page are exact; for anything measured in months or years, treat the result as an average.