The "Object of type 'System.String' cannot be converted to type 'System.DateTime'" error generally occurs due to two possible problems.

1. A value other than a Date has been entered into a Date field. If this error occurs review the date columns on your import sheet and ensure that each value is filled out correctly.
2. Only some of the rows within the Date column are filled out. When this happens the Import Wizard cannot determine what the empty cells are. If this occurs, enter 1/1/1900 into each empty cell in that date column. This is a placeholder date that is used to allow the Import Wizard to function, but this date will not actually show up in MVP.