There should be no difference but Notesdatetime.setnow seems to be more accurate in a timezone sort of way than new notesdatetime(now) .

This:

 Dim ndt As notesdatetime
Set ndt=New notesdatetime("")
Call ndt.setnow
Dim uiw As New NotesUIWorkspace
Dim uidoc As notesuidocument
Set uidoc = uiw.currentdocument

Call uidoc.FieldSetText("subject",ndt.zoneTime)
Msgbox ndt.zoneTime

seems to work better than ( NB is using now legal? )

 Dim ndt As notesdatetime
Set ndt = New notesdatetime(Now)
Dim uiw As New NotesUIWorkspace
Dim uidoc As notesuidocument
Set uidoc = uiw.currentdocument

Call uidoc.FieldSetText("subject",ndt.zoneTime)
Msgbox ndt.zoneTime

Discuss!