Thanks for posting!
At the moment, it's not clear if this is a bug, how it's designed or just a limitation of FileMaker Go. May I ask why a date field wouldn't work for your purposes?
Thanks. We wanted a way for DocuBin users to "create" fields without actually modifying the database schema, so we created a table with two fields: field one holds the value of field 2's data type. Field 2 is text field with multiple occurrences on the layout each with a different control style or field formatting. Depending on which field type field 1 holds determines which layout object for field 2 the user is taken to.