1665 (MDCLXV) was a common year starting on Thursday of the Gregorian calendar and a common year starting on Sunday of the Julian calendar, the 1665th year of the Common Era (CE) and Anno Domini (AD) designations, the 665th year of the 2nd millennium, the 65th year of the 17th century, and the 6th year of the 1660s decade. As of the start of 1665, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.