The Gregorian Calendar:One of the most widely used calendars today is the Gregorian Calendar, named after Pope Gregory XIII, who introduced it in 1582. It replaced the Julian calendar, which had become out of sync with the solar year due to its slightly longer duration. The Gregorian calendar refined the leap year system, ensuring greater accuracy in aligning with the Earth's orbit around the sun. The discrepancy between the solar year and the calendar year, while minimal, accumulates over time, leading to occasional adjustments such as leap seconds. Efforts to reform the calendar have been proposed over the years, ranging from adjustments to the leap year system to the adoption of entirely new calendar models.