How does the Gregorian leap-year rule differ from the Julian leap-year rule?

The Julian calendar and the Gregorian calendar differ in how they account for the length of the solar year to maintain seasonal accuracy.

• Julian Rule: This system assumes a solar year of exactly 365.25 days. Consequently, it designates every fourth year as a leap year without exception.312
• Gregorian Rule: Because the actual tropical solar year is approximately 365.242 days, the Julian system overestimates the year's length, causing a drift of about one day every 130 years. To correct this, the Gregorian calendar drops three leap years every 400 years. Under this rule, a century year (ending in 00) is a leap year only if it is exactly divisible by 400 (e.g., 1600 and 2000 were leap years, while 1700, 1800, and 1900 were not).32

This refinement ensures the calendar remains more closely aligned with the Earth's orbit around the Sun, preventing the seasonal displacement that occurred under the Julian system.31