The reason for changing the calendar was that experts in the early 1700s realised that there is a mistake in the Julian calendar, that it adds too many leap years. This meant that the date of Easter was being calculated wrongly. So they designed a new calendar that corrected this error. Pope Gregory XIII had declared that this new calendar should be used from 1582 onwards. But only Roman Catholic countries accepted this ruling: Protestant and Eastern Orthodox countries refused to have the Pope tell them what to do. So even in Europe, the change happened at different times.