1634 (MDCXXXIV) was a common year starting on Sunday of the Gregorian calendar and a common year starting on Wednesday of the Julian calendar, the 1634th year of the Common Era (CE) and Anno Domini (AD) designations, the 634th year of the 2nd millennium, the 34th year of the 17th century, and the 5th year of the 1630s decade. As of the start of 1634, the Gregorian calendar was 10 days ahead of the Julian calendar, which remained in localized use until 1923.