PROB §4673: (a) A written advance health care directive is legally sufficient if all of the following requirements are satisfied: (1) The advance directive contains the date of its execution.

(a) A written advance health care directive is legally sufficient if all of the following requirements are satisfied: (1) The advance directive contains the date of its execution. (2) The advance directive is signed either by the patient or in the patient’s name by another adult in the patient’s presence and at the patient’s direction. (3) The advance directive is either acknowledged before a notary public or signed by at least two witnesses who satisfy the requirements of Sections 4674 and 4675. (b) An electronic advance health care directive or power of attorney for health care is legally sufficient if the requirements in subdivision (a) are satisfied, except that for the purposes of paragraph (3) of subdivision (a), an acknowledgment before a notary public shall be required, and if a digital signature is used, it meets all of the following requirements: (1) The digital signature either meets the requirements of Section 16.5 of the Government Code and Chapter 10 (commencing with Section 22000) of Division 7 of Title 2 of the California Code of Regulations or the digital signature uses an algorithm approved by the National Institute of Standards and Technology. (2) The digital signature is unique to the person using it. (3) The digital signature is capable of verification. (4) The digital signature is under the sole control of the person using it. (5) The digital signature is linked to data in such a manner that if the data are changed, the digital signature is invalidated. (6) The digital signature persists with the document and not by association in separate files. (7) The digital signature is bound to a digital certificate.