Cal. Code Civ. Proc. § 360.5
No waiver shall bar a defense to any action that the action was not commenced within the time limited by this title unless the waiver is in writing and signed by the person obligated. No waiver executed prior to the expiration of the time limited for the commencement of the action by this title shall be effective for a period exceeding four years from the date of expiration of the time limited for commencement of the action by this title and no waiver executed after the expiration of such time shall be effective for a period exceeding four years from the date thereof, but any such waiver may be renewed for a further period of not exceeding four years from the expiration of the immediately preceding waiver. Such waivers may be made successively. The provisions of this section shall not be applicable to any acknowledgment, promise or any form of waiver which is in writing and signed by the person obligated and given to any county to secure repayment of indigent aid or the repayment of moneys fraudulently or illegally obtained from the county.
Ca. Civ. Proc. Code § 360.5