Rosemary,
As Brandon suggested, you may wanna try this is a test company first..
Blank dates might be related to "Blanket PO's" (?)..
Anything else, I'd try to start running a check-links in the maintenance of GP to see if there are any documents that could be eliminated by the system due to their unclear status.
GP has a function to change status of PO's and cancel them, but you may have to cancel the items individually before. If you have a large numbers of PO's to get rid of, the line item status could be changed from the back-end and then cancel the PO's from the GUI.
Always a good idea to run a reconcile after that.
PS: do you have any external integration that happens to create PO's in GP ? that could explain those "new" PO's without a valid date