Days in a Year

• Standard (non‑leap) year: 365 days
• Leap year: 366 days

Why Some Years Have 366 Days

The Gregorian calendar, which is used by most of the world, adds an extra day (February 29) to keep the calendar in sync with Earth’s orbit around the Sun. Earth’s orbital period is about 365.2425 days, so without correction the calendar would drift over time.

Leap‑Year Rules (Gregorian Calendar)

1. A year that is divisible by 4 is a leap year.
2. Centurial years (those ending in “00”) are not leap years unless they are also divisible by 400.
   • Example: 2000 was a leap year (divisible by 400), but 1900 was not.

These rules give an average year length of 365.2425 days over a 400‑year cycle, matching the Earth's orbital period closely ¹⁴.

Comparison with Other Calendars

• The Julian calendar uses a simpler rule (every 4th year is a leap year) and therefore has an average year length of 365.25 days ¹.

Summary

• Typical year: 365 days.
• Leap year (every 4 years, with century exceptions): 366 days.
• Average length of a Gregorian year: 365.2425 days (≈ 365 days 5 hours 48 minutes).