1643 (MDCXLIII) was a common year starting on Thursday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1643rd year of the Common Era (CE) and Anno Domini (AD) designations, the 643rd year of the 2nd millennium, the 43rd year of the 17th century, and the 4th year of the 1640s decade. As of the start of 1643, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.