(a) The party propounding interrogatories and the responding party may agree to extend the time for service of a response to a set of interrogatories, or to particular interrogatories in a set, to a date beyond that provided in Section 2030.260.
(b) This agreement may be informal, but it shall be confirmed in a writing that specifies the extended date for 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 interrogatory to which the agreement applies in any manner specified in Sections 2030.210, 2030.220, 2030.230, and 2030.240.