Cal. Penal Code § 1331.5

Any person who is subpoenaed to appear at a session of court, or at the trial of an issue therein, may, in lieu of appearance at the time specified in the subpoena, agree with the party at whose request the subpoena was issued, to appear at another time or upon such notice as may be agreed upon. Any failure to appear pursuant to such agreement may be punished as a contempt, and a subpoena shall so state. The facts establishing such agreement and the failure to appear may be shown by the affidavit of any person having personal knowledge of the facts and the court may grant such continuance as may be appropriate.