In 1582 CE, when the Julian calendar was out of synchronization with the solar year by ten days, Pope Gregory ordered a study for its reform. The committee decided that leap years should occur every four years except for three of every four century years ending in 00. Ten calendar days were dropped in October 1582. This made the Gregorian calendar more accurate than the Julian one.
The new calendar was adopted by 1587 in Roman Catholic countries of Europe. It was not adopted in Great Britain until 1752. In that cocuntry, twelve days were dropped in September of that year for synchronization. Greece did not adopt it until 1923. Only Netherlands does not use the Roman-Gregorian names, but uses descriptive Dutch names.
Each day is divided into twenty-four-hour periods, running from midnight to midnight. The day’s length is divided into two units of twelve hours each by some countries and left in one unit of twenty-four hours by others. The number of days in the months was changed from that in the Julian, thus putting the months out of synchronization with the waxing and waning of the moon.