Time Converter

Time units below a month have exact ratios, and above a month they do not. That single split explains everything awkward about converting time.

The exact part is straightforward. Sixty seconds to a minute, sixty minutes to an hour, twenty-four hours to a day, seven days to a week. Ninety minutes is 1.5 hours; 3,600 seconds is one hour; 10,080 minutes is a week. No ambiguity anywhere.

Months and years are the problem. A month is 28, 29, 30 or 31 days depending on which one, and a year is 365 days except when it is 366, so there is only an average. The Gregorian year averages 365.2425 days (365 plus a quarter, minus the century-year correction, plus the four-hundred-year correction).

The conversion that matters commercially is a different one: decimal hours against hours and minutes. Payroll is computed in decimal hours, because a rate is dollars per hour and hours have to be a number you can multiply. A clock reads in hours and minutes. The conversion is one multiplication: take the fractional part and multiply by 60. The reverse divides the minutes by 60.

Only multiples of 3 minutes convert to a terminating decimal. Fifteen minutes is 0.25 and six minutes is exactly 0.1, but twenty is 0.3333 recurring and twenty-five is 0.41666 recurring. That is why timesheet rounding rules exist. Rather than carry recurring decimals, many employers round worked time to the nearest increment. The 15-minute rule is the most common in the US and is lawful only when it rounds both ways; always rounding down is wage theft. Six-minute increments (tenths of an hour) are the other common choice and have the advantage of being exact: every multiple of 6 minutes is a clean tenth.

One further note on the 24-hour day. It is exact as a unit of time and not exactly a rotation of the Earth, which is why leap seconds exist: occasional one-second adjustments to keep clock time aligned with the planet. For any purpose this converter serves, a day is 86,400 seconds; for satellite navigation, it is not quite.