Your When setting needs to point to a date. Not a true/false value. You can then set a filter under the features tab to filter birthdays when date is Within Today.
I did something similar to this solution. Using a simple formula “=MONTH($G$1)=MONTH(F3)”
Where $G$1 is todays date (you could always add the =today() instead but i had thousands of cells referencing this so it was less time consuming.) and the F3 was the birthday. This formula returned a TRUE or FALSE value and my birthday list just filters for TRUE.