You don’t need a today’s date propert for this. Instead create a contact based workflow. In the first step after clicking “Create workflow”, HubSpot lets you choose a workflow type. Choose the one that is centered around a date property, select the birthday date and set it to recurring annually.
@Yi-tzu you created a regular contact based workflow. Please go back, create a new one and on the first selection screen (before you end up in the editor) select a contact based workflow centered around a contact date property.