2016-12-10 Sat

The Gregorian calendar decreed by papal bull Inter gravissimas by Pope Gregory XIII in 1582, is the most commonly used calendar in the world today. It is a solar calendar based on a 365-day common year divided into 12 months of irregular lengths – 11 months having 30 or 31 days and the second month, February, having 28 days (common year) or 29 days (leap year every four years).

Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Italian doctor, astronomer and philosopher Luigi Lilio (also known as Aloysius Lilius). He was born around 1510 and died in 1576, six years before his calendar was officially introduced.

The Gregorian calendar's predecessor, the Julian calendar, was replaced because it was too inaccurate. It did not properly reflect the actual time it takes the Earth to circle once around the Sun, known as a tropical year.

The delay in switching meant that different countries not only followed different calendars for a number of years but also had different rules to calculate whether a year was a leap year. To get the calendar back in sync with astronomical events like the vernal equinox or the winter solstice, a number of days were dropped.

France adopted the new calendar on this date in 1582.

The stamp was issued by Montenegro to commemorate the introduction of the calendar features a watch fob with a map of the Indian Ocean east to the South Pacific, including India, Indo-China and Australia. In the background is "1582" the year the calendar was introduced, "425" the number of years between 1582 and 2007 when the stamp was issued, and "26" the number of seconds the Gregorian calendar differs from the solar calendar in a year.