Located in the Mountain (GMT -07:00) time zone. Area code 303 serves the state of Colorado. This area code was assigned on January 1st, 1947. On January 1st, 1947 it went into service. 303 is a General Purpose Code. 303 service area does overlap with 720 area codes. Dialing pattern for local calls within the same NPA (area code) is 10D.