1645 (MDCXLV) was a common year starting on Sunday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1645th year of the Common Era (CE) and Anno Domini (AD) designations, the 645th year of the 2nd millennium, the 45th year of the 17th century, and the 6th year of the 1640s decade. As of the start of 1645, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.