The Gregorian calendar is the calendar system used by most of the world today. It is a solar calendar that was introduced by Pope Gregory XIII in 1582 as a reform of the Julian calendar. The Gregorian calendar is based on the Earth's revolution around the sun, and a year is defined as the time it takes for the Earth to complete one orbit around the sun. A Gregorian year consists of 365 or 366 days, depending on whether it is a leap year or not. A leap year in the Gregorian calendar occurs every 4 years, except for years that are divisible by 100 but not by 400.
The Muslim calendar, also known as the Hijri calendar, is a lunar calendar that is used by Muslims all over the world to determine the dates of important religious events, such as Ramadan and Eid al-Fitr. The Muslim calendar is based on the cycles of the moon, and a year is defined as the time it takes for the moon to complete 12 lunar cycles of approximately 29.5 days each. A Muslim year is therefore 354 or 355 days long, and it begins with the month of Muharram. The Hijri calendar does not have leap years, but it has a 30-year cycle of alternating 11 and 12 months. The Muslim calendar is based on the migration of the Prophet Muhammad (peace be upon him) from Mecca to Medina, which is known as the Hijrah, and it is therefore also known as the Hijri calendar.