Located in the Central (GMT -06:00) time zone. Area code 312 serves the state of Illinois. This area code was assigned on January 1st, 1947. On January 1st, 1947 it went into service. 312 is a General Purpose Code. 312 service area does overlap with 872 area codes. Dialing pattern for local calls within the same NPA (area code) is 1+10D.