It was reform of the Julian calendar.

The Gregorian reform contained two parts: a reform of the Juliancalendaras used prior to Pope Gregory XIII's time and a reform of the lunar cycle used by the Church, with the Julian calendar, to calculate the date of Easter

It is a reform of the Julian calendar, which loses a day every 128 years. The Gregorian calendar loses a day every 3200 years, making it 25 times more accurate.

The Gregorian calendar is the standard calendar of the "western" world. It was introduced in 1582 as a reform of the Julian calendar, which is almost identical but has 7.5 more leap year days per millennium than the Gregorian calendar, making it about 25 times less accurate.

The Gregorian calendar was an adaptation of a calendar proposed by Aloysius Lilius in 1582. However, the calendar is named after Pope Gregory XIII who introduced this calendar by a papal bull. It was a reform to the Julian calendar.