If I change date format with hook_form_alter
for example from:
$form['field_date']['min']['#date_format'] = "Y-m-d";
to:
$form['field_date']['min']['#date_format'] = "d.m.Y";
views calc not working.

error:
The value input for field Dátum od is invalid:
The value 01.05.2013 does not match the expected format.
The value input for field Dátum do is invalid:
The value 31.05.2013 does not match the expected format.