Who developed the calendar system we use today?

The calendar system widely used today, known as the Gregorian calendar, was developed by Pope Gregory XIII in 1582. It was introduced to reform the earlier Julian calendar, which had inaccuracies in calculating the length of the solar year. The Gregorian calendar corrected these discrepancies by adjusting the leap year system, leading to a more accurate alignment with the seasons. It has since been adopted by most countries around the world.