Pivot Table - Filter DOB field by Month not Year

Pivot Table - Filter DOB field by Month not Year

Form "Employee" includes a DOB field, and a "Manager" field.

I need a pivot table to show, for each unique Manager, a list of all their employees with birthdays in each month.

I want to apply a User Filter for This Month, and Next Three Months.

How do I get the DOB to filter by the month only, and not the year??