Decimal Clock

Common questions

What is decimal time?

It is the day divided by tens instead of by sixties. The French Republic decreed it in 1793: 10 hours in a day, 100 minutes in each hour, 100 seconds in each minute, which makes 100,000 decimal seconds where the ordinary clock has 86,400. One decimal second is therefore exactly 0.864 of an ordinary second, and one decimal hour lasts 2 hours 24 minutes. Official use ran for a few months from late 1794 and was suspended in April 1795, though clockmakers kept producing dials with both scales.

Why does a decimal clock show 5:00:00 at noon?

Because noon is half the day gone, and half of 10 decimal hours is 5. Midnight at the start of the day is 0:00:00, six in the morning is 2:50:00, six in the evening is 7:50:00, and the last ordinary second of the day, 23:59:59, reads 9:99:98. There is no hour 10 on the dial: the clock rolls back to 0:00:00 at the next midnight. The two landmark rows under the clock on this page show midnight and noon at all times, so the scale is always readable at a glance.

How do you convert a normal time to decimal time?

Count the seconds since midnight and divide by 0.864. Six in the evening is 18 x 3,600 = 64,800 seconds, and 64,800 / 0.864 = 75,000 decimal seconds, which is 7:50:00. Going the other way, multiply the decimal seconds by 0.864. The clock reading here truncates rather than rounds, so it never shows a decimal second before it happens; the timesheet figure beside it is rounded to three decimals, which is 3.6 seconds.

Is 18.5 hours the same as 18:50?

No, and this is the mistake the page is built to prevent. 18.5 is a decimal hour in the timesheet sense: the ordinary 24-hour clock with the minutes written as a fraction, so 18.5 hours means 18:30. Payroll and billing systems mean this when they say decimal, while a decimal clock means the ten-hour day, where the same moment reads 7:70:83. Both readings are computed here for every time, each one labelled, and the Scale control only decides which is shown large.

Does it use my time zone, and what happens when the clocks change?

It reads the instant from your device and converts it into whichever zone you select, using your browser's own time zone database, so the offset is right for the date rather than taken from a fixed table. Decimal time here follows the wall clock in that zone, which means that on the two days a year the clocks move, the civil day is 23 or 25 hours long and the decimal reading jumps or repeats with it. The page says which, names the hour it happens at, and flags a typed time that the day skips or runs twice.

Can I convert a particular time rather than watch the clock?

Yes. Switch the clock control to Set time and type the time in almost any form: 18:31:40, 18:31, 1831, 183140, 6:31:40 pm, or the words noon and midnight. Anything it cannot read is refused by name rather than guessed at, so an hour past 23, a minute past 59 or a bare 18.5 each get their own message saying what is wrong and what to type instead.

Does the clock send anything anywhere?

No. Every conversion happens in the page, there is no account and nothing is uploaded. Your scale, zone and typed time are remembered in this browser only, and only once you change a control; the Start over button above the tool forgets them again.