The answer is found at DFARS 217.204: "...the ordering period of a task order or delivery order contract (including a contract for information technology) awarded by DoD pursuant to 10 U.S.C. 2304a—

     (A) May be for any period up to 5 years;

     (B) May be subsequently extended for one or more successive periods in accordance with an option provided in the contract or a modification of the contract; and

     (C) Shall not exceed 10 years unless the head of the agency determines in writing that exceptional circumstances require a longer ordering period.

Task orders may be performed up to five years (one-year base plus four one-year options) past the basic contract expiration date only when the order is issued before the basic contract expiration date."

In other words, the task order itself can add a period of performance of up to five years past the expiration date of the basic contract. Note that DFARS 217.204 contains some exceptions to this authority.