Almost everything in modern measurement is decimal, except time. A day has 24 hours, an hour 60 minutes, a minute 60 seconds. The reasons go back thousands of years, to Egyptian astronomers and Babylonian mathematicians.
Twelve hours of night, twelve of day
Ancient Egyptians tracked the night by watching groups of stars rise, and divided the night into twelve parts. Daylight, measured with sundials, was also divided into twelve. Together they gave a day of 24 hours. These early hours were not equal: a summer daylight hour was longer than a winter one, because the daylight itself was longer.
Sixty from Babylon
The 60s come from Mesopotamia, where Babylonian mathematicians counted in base 60. Sixty divides evenly by 2, 3, 4, 5, 6, 10, 12, 15, 20 and 30, which makes fractions easy. Greek astronomers inherited the system for dividing circles and angles into degrees, minutes and seconds, and applied the same divisions to the hour. The words minute and second come from Latin for the “small” division and the “second” small division.
Equal hours and the second
Hours of equal length became normal with mechanical clocks in 14th-century Europe. Minutes and seconds were mostly theoretical until Christiaan Huygens built the first pendulum clock in 1656, accurate enough to make the second useful. For centuries the second was defined as a fraction of the average day.
Atomic time
Earth’s rotation is not perfectly steady, so in 1967 the second was redefined using atoms: the duration of 9,192,631,770 oscillations of radiation from cesium-133 atoms. Atomic clocks are now so precise that they show Earth’s day varies by milliseconds.
To keep clocks in step with the sun, 27 leap seconds were inserted between 1972 and 2016. They cause problems for computer systems, and in 2022 the international measurement conference decided to stop adding them by 2035. The time converter counts a day as exactly 86,400 seconds.
Weeks, months and years
The seven-day week traces back to ancient Mesopotamia and the Jewish Sabbath, and was adopted across the Roman Empire in the 4th century. Months and years follow the moon and the sun, so their lengths vary: a month is 28 to 31 days and a calendar year 365 or 366. That is why the converter offers weeks but not months or years: they have no single length in seconds.
Decimal hours
Timesheets often show decimal hours, which mixes the two worlds: 1.5 hours is 1 hour 30 minutes, not 1 hour 50 minutes, and 0.25 hours is 15 minutes. The converter’s breakdown always shows the clock form, so you don’t have to do the base-60 math yourself.
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- Time converter — seconds to weeks, on a clock you can replay