Navigate to:
Admin Center -> SharePoint -> More features -> Search
Manage Search Schema -> Managed Properties

Search for: refinabledate00 -> open
add: Mappings to crawled properties: People:SPS-Birthday

Search for: refinabledate01 -> open
add: Mappings to crawled properties: People:SPS-HireDate

It will take a couple minutes to a couple hours / Days until the Search database is updated with the new properties.


WebPart:
Use the Fast Search properties within the Birthday Reminder WebPart Properties:
SPS-Birthday Managed Prop.Name: RefinableDate00
SPS-HireDate Managed Prop. Name: RefinableDate01

To use the values within the FieldTemplate, just replace the default property {SPS-Birthday} or {SPS-HireDate} with its {RefinableDate00} or {RefinableDate01}