nt:hasDatatype on a literal placeholder both constrains the value and changes the field. With xsd:date or xsd:dateTime, the form shows a date picker instead of a text field. For a point in time coarser than a day, the types xsd:gYear, xsd:gYearMonth, xsd:gMonth, xsd:gMonthDay and xsd:gDay are offered as parts to pick, such as a year alone.

The datatype is matched exactly. A placeholder with a datatype does not accept a plain, untyped value, so adding a datatype to a field of an existing template makes the older nanopublications, whose values are untyped, no longer fit it.