The date of separation in CA is the date the parties physically separated; this is generally when one party moves out, or when they hold themselves out to the community as no longer a couple.
A separated spouse can still be found liable for the "necessities" of life, which generally includes mandatory medical procedures. However, the creditors will first pursue collection from the estate of the debtor unless the separated spouse cosigned on the bills. Another way to avoid payment is to either secure a judgment of dissolution and have the debts assigned to the incurring spouse; or to have a written or verbal agreement pursuant to the last referenced code, stating that the spouses are not liable for the support of one another.
Here is a case on point that found the separated spouse was not liable for medical debt incurred during separation: CMRE Financial Services, Inc. v. Parton
Here are the relevant codes:
913. (a) The separate property of a married person is liable for a debt incurred by the person before or during marriage. (b) Except as otherwise provided by statute: (1) The separate property of a married person is not liable for a debt incurred by the person's spouse before or during marriage. (2) The joinder or consent of a married person to an encumbrance of community estate property to secure payment of a debt incurred by the person's spouse does not subject the person's separate property to liability for the debt unless the person also incurred the debt. 914. (a) Notwithstanding Section 913, a married person is personally liable for the following debts incurred by the person's spouse during marriage: (1) A debt incurred for necessaries of life of the person's spouse while the spouses are living together. (2) Except as provided in Section 4302, a debt incurred for common necessaries of life of the person's spouse while the spouses are living separately. (b) The separate property of a married person may be applied to the satisfaction of a debt for which the person is personally liable pursuant to this section. If separate property is so applied at a time when nonexempt property in the community estate or separate property of the person's spouse is available but is not applied to the satisfaction of the debt, the married person is entitled to reimbursement to the extent such property was available. (c) (1) Except as provided in paragraph (2), the statute of limitations set forth in Section 366.2 of the Code of Civil Procedure shall apply if the spouse for whom the married person is personally liable dies. (2) If the surviving spouse had actual knowledge of the debt prior to expiration of the period set forth in Section 366.2 of the Code of Civil Procedure and the personal representative of the deceased spouse's estate failed to provide the creditor asserting the claim under this section with a timely written notice of the probate administration of the estate in the manner provided for pursuant to Section 9050 of the Probate Code, the statute of limitations set forth in Section 337 or 339 of the Code of Civil Procedure, as applicable, shall apply.
4302. A person is not liable for support of the person's spouse when the person is living separate from the spouse by agreement unless support is stipulated in the agreement.