Revert #c8901abc and fix duplicate test condition.

src/@dates/setdiff.m

@@ -27,7 +27,7 @@ function [q, io] = setdiff(o, p)
 % You should have received a copy of the GNU General Public License
 % along with Dynare.  If not, see <https://www.gnu.org/licenses/>.
 
-if ~isa(o, 'dates')
+if ~isa(o, 'dates') || ~isa(p, 'dates')
     error('dates:setdiff', 'All input arguments must be dates objects.')
 end