The Gregorian calendar, Western calendar or the Christian calendar, is a calendar that was a reform in 1582 to the Julian calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named. But was is an adaptation of a calendar designed by Italian doctor, astronomer and philosopher Luigi Lilio. And it is not nesassarly based of Jesus birth , most scholars arrive at a date of birth between 6 and 4 BC. 
The Gregorian reforms were never intended to unify the calendars of Europe, just those of the Christian nations. However, the Inter Gravissimas began a chain of events that are still ongoing in the world, over four hundred years later, and the Gregorian Calendar has ceased to be a primarily Christian institution. Although the Gregorian calendar slowly spread across the world during the 18th and 19th centuries, the situation in Europe remained static until 1912. The event that united the calendars of Europe was the 1st World War. Some countries were forced into accepting the reforms by an occupying army, like Latvia and Lithuania - while others, notably Russia, switched in the aftermath of a revolution. The post-war division of the Ottoman Empire led to many of the Balkan countries adopting the Gregorian calendar, and Turkey in 1926.
So from that point on Europe, America and France being the most "civilized and advanced nations " after the war it slowly became internationally the most widely used civil calendar. It has been the unofficial global standard for decades, recognised by international institutions like the United Nations and the Universal Postal Union. So by default because of other advancing nations needing to be on the "same page " as it has already been established universally, they adopt it also.
Although not all countries have adopted the Gregorian calendar Saudi Arabia, Ethiopia, Nepal, Iran and Afghanistan. Some countries use other calendars alongside the Gregorian calendar, including India, Bangladesh, Israel and Burma; and other countries use a modified version of the Gregorian calendar, including Sri Lanka, Cambodia, Thailand, Japan, North Korea and Taiwan. The Eastern Orthodox Church has never fully accepted the calendar.
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