1900 (MCM) was an exceptional century common year starting on Monday of the Gregorian calendar and a leap year starting on Saturday of the Julian calendar, the 1900th year of the Common Era (CE) and Anno Domini (AD) designations, the 900th year of the 2nd millennium, the 100th and last year of the 19th century, and the 1st year of the 1900s decade. As of the start of 1900, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.