Simplest way would be to remove the empties from the found set. Or if you are viewing in a portal, filter the portal to show only records that have a date.
I agree with keywords, but if you must have them on the list, and I can imagine reasons why you might:
Define a calculation field with a date result type as:
If ( IsEmpty ( Estimated ship Date ) ; Date ( 12 ; 12 ; 3000 ) ; Estimated Ship Date )
Sort your records on this field instead of estimated ship date. This field need not be located on the layout.
I was came up with the same solution just before you posted.
Yes I need these to show up in this particular layout.