California Code, Penal Code - PEN § 1237.1

No appeal shall be taken by the defendant from a judgment of conviction on the ground of an error in the calculation of presentence custody credits, unless the defendant first presents the claim in the trial court at the time of sentencing, or if the error is not discovered until after sentencing, the defendant first makes a motion for correction of the record in the trial court, which may be made informally in writing.  The trial court retains jurisdiction after a notice of appeal has been filed to correct any error in the calculation of presentence custody credits upon the defendant's request for correction.