...

src/calibre/gui2/dialogs/search.py

@@ -39,6 +39,7 @@ class SearchDialog(QDialog, Ui_Dialog):
         for val, text in [('today', _('Today')), ('yesterday', _('Yesterday')), ('thismonth', _('This month'))]:
             self.date_human.addItem(text, val)
         self.date_year.setValue(now().year)
+        self.date_day.setSpecialValueText(u' \xa0')
         vals = [((v['search_terms'] or [k])[0], v['name'] or k) for k, v in db.field_metadata.iteritems() if v.get('datatype', None) == 'datetime']
         for k, v in sorted(vals, key=lambda (k, v): sort_key(v)):
             self.date_field.addItem(v, k)

src/calibre/gui2/dialogs/search.ui

@@ -424,9 +424,6 @@
          </item>
          <item>
           <widget class="QSpinBox" name="date_day">
-           <property name="specialValueText">
-            <string>  </string>
-           </property>
            <property name="maximum">
             <number>31</number>
            </property>