This workflow shows how dates can be read from contacts. The Get Details of Contacts action does not (as of the time of writing) allow custom date fields to be read in from a contact. Instead you must use the Get Dates from Input action. In the workflow the user first selects a contact (ideally with some custom date fields populated) and the workflow then parses the dates returned from the contact - selecting the names of the fields using the Get Name action.
The user is then presented with a list of the available fields and can select one of these to be retrieved. Once selected Workflow parses the list of dates again until it finds one with a matching name (to that selected by the user). The workflow then retrieves the date associated with that field and formats it into the standard short date format.
This is finally returned to the user via a Quick Look action.