Modifying ticket tag for display

I am currently using the following display the time left before delivery (from a ticket tag) on my KDS… I want to show the delivery time MINUS 15 minutes so my kitchen staff hurries based on the timer and leaves enough time for the driver.

Is there any way to modify this in real time for the display?

<L00><size 15><timer {TICKET DATE:HH:mm} {TICKET TAG:Delivery Minutes} 1>

Basically I want to display the value of TICKET TAG: Delivery Minutes but minus 15 minutes.

Thank you.