Hello @interioryardage thanks for your post. And welcome to the Square Community! We're glad you're here!
If you've been collecting birthdays, have you done so by using a Marketing automation? There are different automatic emails you can setup to send to your customers; one is for the birthday collection, and another is to send an actual offer to birthdays. I think the emails are sent by the month, so everyone with a birthday in that month will get an email. You can customize the email and the offer they get emailed. I think you'd be able to specify the 15% discount, and this may/or may not be applied to any item.
Go to your Square Dashboard > Marketing > Automations to set up this birthday collection/offer email. I hope this helps answer your question.
