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1800 (MDCCC) was an exceptional common year starting on Wednesday in the Gregorian calendar and a leap year starting on Sunday in the Julian calendar. As of the start of 1800, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.

As of March 1 (O.S. February 18), when the Julian calendar acknowledged a leap day and the Gregorian calendar didn't, the Julian calendar fell one day further behind, bringing the difference to 12 days until February 28 (O.S. February 16), 1900.