DXL: Item with type="400" in export?

When dxl exporting a field of type date with a value of 12:00am, I get an item of type=“400” and its value is:

AAAAAP////8=

with an additional CRLF (or similar) at the end of the value.

I believe the date item may have effectively been set using:

set dt=new NotesDateTime(Cdat(0))

set itm=doc.replaceItemValue(fieldName,“”)

set itm.dateTimeValue=dt

Has anyone else seen something like this?