Good information @merahza...I listened to the information about the origin of the Islamic calendar ... I listened to Ustad's lecture in his lecture to speak about it.

The Gregorian calendar was a reform of the Julian calendar. It was instituted in 1582 by Pope Gregory XIII, after whom the calendar was named, by papal bullInter gravissimas dated 24 February 1582.[4] The motivation for the adjustment was to bring the date for the celebration of Easter to the time of year in which it was celebrated when it was introduced by the early Church.