You need to ensure the Custom Document Property type matches your content control's output. A date output as "Tuesday, 5 February" is text, whilst "5/2/2019" is a date. If using a date type, you'll also need code for handling your content control when it displays the placeholder text. For example:
Code:
With ContentControl
If .ShowingPlaceholderText = True Then
ActiveDocument.CustomDocumentProperties("BidO").Value = "1/1/1900"
Else
ActiveDocument.CustomDocumentProperties("BidO").Value = ContentControl.Range.Text
End If
End With