California Code, Labor Code - LAB § 4154

(a) Such employer is subject to the compensation provisions of this division and;

(b) Such person in his service has not, either upon entering into the employment, or within five days after the filing of an election by the employer, given to such employer notice in writing that he elects not to be subject to the compensation provisions of this division.

In case of such acceptance, the person employed becomes subject to the compensation provisions at the time of the filing of the election or entry in the employment.