Decimal time

Decimal time is a way to measure the time of day using units that are all connected by tens, much like counting by tens. This idea was used during the French Revolution when France created a new calendar and time system from 1794 to 1800. In this system, one whole day was split into 10 decimal hours. Each decimal hour was then split into 100 decimal minutes, and each decimal minute into 100 decimal seconds. This means there were 100,000 decimal seconds in one day, instead of the usual 86,400 seconds we use today.

One big advantage of decimal time is that because everything is divided by ten, it is easier to understand and work with the numbers. For example, 1 hour, 23 minutes, and 45 seconds in decimal time would simply be written as 1.2345 decimal hours, or 123.45 decimal minutes, or 12345 decimal seconds. This makes converting times much simpler. It also makes it easy to show the time as a part of the whole day, like writing the date as 2026-05-19.54321 to mean five decimal hours, 43 decimal minutes, and 21 decimal seconds after midnight.

Decimal time also fits well with computers and digital systems because the numbers can be used directly for calculations and displays without extra steps. This system shows how useful it can be to have a consistent base for measuring and calculating time.

History

Egypt

Main article: Decan

The ancient Egyptians used groups of stars called decans to help divide the 360 degrees of the sky into 36 parts of 10 degrees each. Every ten days, a new group of these stars would appear in the sky just before the sun rose. This helped them track time over a full year of 365 days.

China

Main article: Traditional Chinese timekeeping

In China, people used a special way to tell time for most of history. They split the day into different parts, sometimes using 12 parts and sometimes using 10 parts called shi and 100 smaller parts called ke. Over time, they even split these tiny parts into 100 even smaller pieces called fen and then into 100 tiny pieces called miao. This made timekeeping very exact!

France

See also: French Republican calendar

Pre-Revolution

Even before big changes in France, people thought it would be easier to split time into parts that are all tens, like 10 hours in a day instead of 24, and 100 minutes in an hour instead of 60. This would make math with time much simpler.

French Republic

During a big change in France called the French Revolution, leaders decided to try out this new way of telling time. They said a day would be split into 10 hours, each hour into 100 minutes, and each minute into 100 seconds. Midnight was called "ten hours" and noon was called "five hours."

Representation

Back then, people didn’t use the colon (:) to show time like we do today. Instead, they might write things like "5 hours 83 minutes" to show a certain time. They also used special words like décimes for ten-minute parts of an hour.

Usage

Even though clocks were made to show this new time, most people stuck with the old way of telling time. The new system was only used for a short time and then stopped. But scientists still found it useful for their work.

Metric system

When France was creating a new system to measure things, they thought about using this decimal way of telling time too. But they decided to keep the old way for most everyday use, even though scientists still find it helpful.

Swatch Internet Time

Main article: Swatch Internet Time

In 1998, a Swiss watch company called Swatch tried another new way to tell time. They split the day into 1,000 parts called “beats.” Each beat was a little less than a minute in our normal time. Midnight was “@000” and noon was “@500.” But this idea didn’t catch on either, and Swatch stopped making watches with this system.

Conversions

In regular time, a day has 86,400 seconds. But in the special French decimal time system, a day had 100,000 decimal seconds. This means each decimal second was a little shorter than a regular second.

Decimal hours

Another way to tell time using decimals is called decimal hours. In 1896, a man named Henri de Sarrauton suggested splitting each day’s 24 hours into 100 smaller parts, called decimal minutes, and each of those into 100 even smaller parts, called decimal seconds. This idea didn’t catch on, but we now often use parts of an hour to show time, which makes counting easier.

Decimal hours are used a lot in jobs that need to count hours worked, like in payroll or billing. Time clocks often record time in parts of an hour, so 08:30 would be written as 08.50. This helps avoid switching between minutes and hours. In aviation, using decimal hours also makes adding up flight times simpler. For example, instead of adding 1 hour and 36 minutes to 2 hours and 36 minutes, you would add 1.6 to 2.6 and get 4.2 hours.

Time clocks

Fractional days

Sometimes, scientists and computers show the time of day as a part of a whole day. For example, midnight is shown as 0.0 day, and noon is shown as 0.5 day. This way, they can easily add the time to any date.

Astronomers often use this method to record when they see things in the sky. They have used it since the 1700s. For example, they might say something happened at 0.5 day, which means exactly halfway through the day. This makes it easier to do math with time because they only need to think about days instead of hours, minutes, and seconds.

Decimal multiples and fractions of the second

The second is the basic unit of time used in science and many computer systems. We can talk about smaller pieces of a second, such as milliseconds, microseconds, and nanoseconds.

Sometimes, scientists use bigger units like kiloseconds or megaseconds to talk about longer periods of time, but we usually just use minutes, hours, days, and years instead. You could measure the time of day in kiloseconds since midnight—for example, 56.7 kiloseconds instead of 3:45 p.m.—but people almost never do this in everyday life.

Scientific decimal time

Scientists often use decimal time, which means dividing time into parts that are easy to work with. For example, a decimal day splits the day into 10 equal parts, and a decimal year splits the year into 10 equal parts. This is simpler than using minutes and seconds, which follow a different counting system, or months and days, which have different lengths.

In astronomy, scientists use something called the Julian day, which measures days in a decimal way from a specific point in time. This helps make calculations easier and more consistent.

Other decimal times

Many people have suggested new ways to measure time using decimal numbers. These ideas divide the day into different parts, making it easier to switch between them. For example, you might see the day shown as 0.500 of a full day, or as 5 decimal hours, or even as 500 millidays.

Some ideas for decimal time are based on special units for measuring time intervals, like those used on a stopwatch, instead of the usual clock time. In stories like Star Trek, they use something called a stardate, which mixes two types of decimal time together.