Why were 1700, 1800, and 1900 not leap years in the Gregorian calendar?

The Julian calendar assumed a solar year of 365.25 days, which is slightly longer than the actual tropical solar year of approximately 365.242 days. This discrepancy caused the calendar to drift by roughly one day per century, leading to a significant misalignment with the seasons over time.123

To correct this, the Gregorian calendar introduced a more precise rule for leap years. While most years divisible by 4 are leap years, century years (those ending in 00) are only considered leap years if they are exactly divisible by 400.123

Because 1700, 1800, and 1900 are not divisible by 400, they are designated as common years rather than leap years. This adjustment effectively drops three leap days every 400 years, ensuring the calendar remains closely synchronized with the solar year.13