Is a defendant entitled to a credit beginning with the date of the crime?

Third, as the People point out, calculation of presentence custody credits is governed by statute. It is a simple matter of mathematics, rather than a matter upon which a trial court, or the People in negotiating a plea agreement, can exercise discretion. (People v. Aguirre (1997) 56 Cal.App.4th 1135, 1139. In other words, neither the People nor the trial court had the authority to promise defendant that his presentence custody credits would begin on the date of the offense if he was at that time incarcerated on another matter. It appears from the record that defendant was indeed incarcerated on another matterthe offense took place while defendant was in prison. Allowing defendant's presentence custody credits to be calculated beginning with the date of the offense would have resulted in an unauthorized sentence under both statute and case law. "A defendant is not entitled to presentence custody credits when he or she is charged with a crime while already incarcerated and serving a sentence on a separate, earlier crime. [Citations.] The test is whether the defendant would have been free 'but for' his or her

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incarceration on the second crime." (People v. Gisbert (2012) 205 Cal.App.4th 277, 281; See also 2900.5, subd. (b) ["For the purposes of this section, credit shall be given only where the custody to be credited is attributable to proceedings related to the same conduct for which the defendant has been convicted."].) Thus, defendant has not established the calculation of presentence custody credits beginning with the date of the offense was a condition of his plea agreement.