The Gregorian calendar is the most widely used civil calendar in the world. A regular Gregorian year consists of 365 days.
A leap year (also known as an intercalary year or bissextile year) occurs every 4 years, except for years that are divisable by 100 and not divisable by 400.
A leap year contains one additional day (February 29) that is added to the calendar in leap years as a corrective measure, because the Earth doesn’t orbit the sun in precisely 365 days.
Julian year is the time interval that is used for astronomical calculations. One julian year = 365.25 days.
One sidereal year is the time that the Earth takes to make one full circle around the Sun.
One sidereal year = 365.25636 days.
Tropical year (also known as a solar year) is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth. One tropiical year = 365.242189 days.