California Code, Code of Civil Procedure - CCP § 2031.270

(a) The party demanding inspection, copying, testing, or sampling and the responding party may agree to extend the date for the inspection, copying, testing, or sampling or the time for service of a response to a set of demands, or to particular items or categories of items in a set, to a date or dates beyond those provided in Sections 2031.030, 2031.210, 2031.260, and 2031.280.

(b) This agreement may be informal, but it shall be confirmed in a writing that specifies the extended date for inspection, copying, testing, or sampling, or for the service of a response.

(c) Unless this agreement expressly states otherwise, it is effective to preserve to the responding party the right to respond to any item or category of item in the demand to which the agreement applies in any manner specified in Sections 2031.210, 2031.220, 2031.230, 2031.240, and 2031.280.