There is only one "Date" in Lr.
If you want to have the date the photo was taken instead of the scan date you have to <Edit Capture Time> via Library Module / Metadata / Edit Capture Time.
See also this thread: http://forums.adobe.com/message/2594805#2594805
Also, I think there is a plug-in that extends the metadata fields and lets you enter the "photo taken" date in addition to "Capture Time".
Check the websites of John Beardy and Rob Cole. Both are contributors on this forum.
