subperiod method is not available for daily/annual frequencies.

src/@dates/subperiod.m

 function s = subperiod(d)
 
-% Returns the subperiod (half-year, quarter, month or week depending on the frequency).
+% Returns the subperiod (half-year, quarter, or month depending on the frequency).
 
-% Copyright (C) 2016-2020 Dynare Team
+% Copyright © 2016-2020 Dynare Team
 %
 % This code is free software: you can redistribute it and/or modify
 % it under the terms of the GNU General Public License as published by
@@ -17,4 +17,8 @@ function s = subperiod(d)
 % You should have received a copy of the GNU General Public License
 % along with Dynare.  If not, see <http://www.gnu.org/licenses/>.
 
+if freq==365 || freq==1
+    error('Method not available for daily or annual frequencies.')
+end
+
 s = d.time(2);
\ No newline at end of file