Located in the Mountain (GMT -07:00) time zone. Area code 720 serves the state of Colorado. This area code was assigned on March 3rd, 1998. On June 1st, 1998 it went into service. 720 is a General Purpose Code. 720 service area does overlap with 303 area codes. Dialing pattern for local calls within the same NPA (area code) is 10D.