The Islamic calendar or Muslim calendar or Hijri calendar is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in many Muslim countries (concurrently with the Gregorian calendar), and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals.This is why Ramadan or the ninth month falls on different days each year a little bit looser than the Christian season of Lent also based on lunar calculations.
The first year was the year during which the emigration of the Islamic prophet Muhammad from Mecca to Medina, known as the Hijra, occurred. Each numbered year is designated either H for Hijra or AH for the Latin anno Hegirae (in the year of the Hijra).
Being a purely lunar calendar, it is not synchronized with the seasons. With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately each 33 Islamic years.
I would assume that the Gregorian calendar is used for global scales in Muslim countries. Seems a bit of a time warp but we in the west look at everything through different eyes sometimes.