Time units below a week are fixed multiples of the second: 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day and 7 days in a week. Months and years are different, because calendar months run 28 to 31 days and years 365 or 366. This converter uses the averages of the Gregorian calendar, 365.2425 days per year and one twelfth of that (30.436875 days) per month, so long spans come out right on average.
Use the converter above for any pair. The most searched pairs have their own pages with worked examples and reference tables, listed above. For the exact gap between two calendar dates, use the date difference calculator instead.