1728 (MDCCXXVIII) was a leap year starting on Thursday of the Gregorian calendar and a leap year starting on Monday of the Julian calendar, the 1728th year of the Common Era (CE) and Anno Domini (AD) designations, the 728th year of the 2nd millennium, the 28th year of the 18th century, and the 9th year of the 1720s decade. As of the start of 1728, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.