For international communications, the whole world does use the Gregorian calendar, but other religious and traditional calendars are still in use around the world. The Jewish calendar has a year that varies between 353 and 385 days. The Muslim calendar has 354 or 355 days in a year.
Religious, administrative and social functions in any country or organization requires a systematic organization of days in the form of a calendar. Calendars designate dates to particular days. Civil calendars are utilized for purposes that are administrative and official in a nation. More often than not, the civil calendars are also used by the general population in their day to day planning. The Gregorian calendar, used across the globe, is believed to be the most extensively relied on civil calendar. Many countries, Christian or not, all over the world have adopted this civil calendar despite its strong association with the Catholic Church. There however are countries which have their own calendars for civil reasons. These countries completely rely on a completely different calendar, either alongside the Gregorian one or in entirety. Some of these civil calendars are a mere modification of the Gregorian calendar.
Iran and Afghanistan share the Solar Hijri calendar for administrative and religious purposes. They do not use the Gregorian calendar at all. This is the same case with Nepal and Ethiopia which exclusively use the Vikram Samvat calendar and the Ethiopian calendar respectively. The Ethiopian calendar has a close correlation with the Egyptian calendar from which it is derived. It is a solar calendar with 29th August or 30th August as the date the year begins in the Ethiopian calendar. Between the Gregorian and the Ethiopian calendars, there exists a 7 to 8 years gap.
For ease of understanding throughout the world, there exists a means of expressing dates from these civil calendars relative to the Gregorian calendar dates.
Picture Credit : Google