Our current calendar is named after a pope, but it all began with the most famous leader of the Roman empire.
Julius Caesar instituted the model for what our calendar is based on in 45 B.C. when he approved what was called the Julian calendar.
According to infoplease.com, that calendar consisted of 11 months of 30 or 31 days each with 28 days in February. Quite accurate for its time, but with a major flaw.
The calendar was off from the real solar year (the time it takes the Earth to circle the sun) by 11 minutes. Over the centuries, that added up.
By the late 1500s, the Julian calendar was off by 10 days from the solar year. So, in 1582, Pope Gregory XIII approved a new calendar that include 365 days and a leap year every four years.
To catch up with the sun, the pope ordered the calendar to advance by 10 days. That officially happened on Oct. 15, 1582.
Not every country immediately accepted the pope's decree and adopted the Gregorian calendar.
It took the British Calendar Act of 1751 to bring England and the American colonies into the fold. By then, the calendar was off by 12 days.
That adjustment was made the following year. In Britian and the colonies, citizens went to sleep on Sept. 2, 1752 and woke up to Sept. 14, 1752.
Other countries were even later. The Netherlands adopted the Gregorian calendar in 1698. Russia accepted it after the 1918 revolution. Greece came on board in 1923.
The Gregorian calendar is only 26 seconds different from a true solar year. That adds up to only one day every 3,323 years.
Many Orthodox churches still use the Julian calendar and they are now 13 days behind the Gregorian calendar.
Many Muslim cultures still use a lunar calendar based on the phases of the moon.
According to infoplease.com, ancient people drew up calendars to coincide with natural phenomena they observed such as phases of the moon and changes in the weather.
Some of our modern time categories are natural. A year is based on the Earth's revolution around the sun. A day records one rotation of our planet.
However, sequences such as months, weeks, hours, minutes and seconds are time divisions chosen by humans.
According to historyworld.net, the numbers we use today were chosen for their divisibility. For a day, 24 hours was chosen because 24 is divisible by a lot of other numbers. In addition, 12 hours was a convenient way to keep of track from dawn to dusk. Noon was the midpoint, or six hours on either side.
For an hour, 60 minutes was chosen, along with 60 seconds, because it too has a lot of factors.
Clocks that recorded minutes were first produced in the 14th century. Clocks that recorded seconds came 200 years later.
The origin of the seven-day week is less certain. According to webexhibits.org, some scholars believe it's based on the creation of the Earth in the Bible where it says God worked for six days and rested on the seventh.
Others, however, say the seven-day week predates the Bible. Theories include ancient people basing the week on the seven known celestial bodies in ancient times or that seven days is close to one moon phase.