Mit "date_1" kann Notes nichts anfangen, ein Wunder dass es überhaupt was zurückliefert.
Korrekt wäre:
Sub Click(Source As Button)
Dim uiws As New notesuiworkspace
Dim uidoc As notesuidocument
dim doc as NotesDocument
Set uidoc = uiws.currentdocument
set doc = uidoc.Document
Dim todaynow As New notesdatetime(doc.GetItemValue("date_1_1")(0))
Dim doccreated As New notesdatetime(doc.GetItemValue("date_1")(0))
Dim difference As Double
difference = doccreated.timedifferencedouble(todaynow)
Msgbox difference
End Sub