These allow the easy reading or writing of these sub-field values of the
user.birthdate property. When migrating from the legacy formats (e.g. MM/DD/YY
or DD/MM/YY), it's required to write all 3 properties to get a correct
birthdate/age. Otherwise, "13/31/69" could become "19691/69" (for example)
which isn't going to parse correctly.