California Code, Code of Civil Procedure - CCP § 1985.1

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 by the court issuing the subpoena. The facts establishing or disproving such agreement and the failure to appear may be proved by an affidavit of any person having personal knowledge of the facts.