What is a Year?

The year equals exactly 365.25 days (the Julian year used in astronomy) — the time for Earth to complete one orbit around the Sun. The Gregorian civil year averages 365.2425 days, achieved through the leap-year rule (every 4 years, except centuries not divisible by 400). The year is the fundamental unit for age, contracts, education, taxation, and astronomical calculation. The 'sidereal year' (Earth's orbit relative to fixed stars) is slightly longer at 365.256 days, while the 'tropical year' (relative to the seasons) is 365.2422 days. The year relates to the day (365.25 days), the month (12 months), and the second (about 31.557 million s). Light-year calculations use the Julian year of exactly 365.25 days. Earth's orbital period has been almost perfectly stable for millions of years, making it a reliable timekeeping reference.

Visual reference for how the year relates to other time units. Each row links to the full converter for that pair.

• Age, anniversaries and legal tenure
• Interest rate calculations
• Astronomy and science

• ✓Julian year = 365.25 days = 31,557,600 seconds.
• ✓Gregorian civil year averages 365.2425 days (97 leap days in 400 years).
• ✓Astronomical "year" can be tropical (365.2422 d), Julian (365.25 d) or sidereal (365.2564 d).

• ✗Using 360 days/year (financial convention) in scientific calculations.
• ✗Confusing Julian and tropical years in precise astronomy.
• ✗Mixing fiscal year (varies by country) with calendar year.