You should not switch the date format, the date format 'mm/yyyy' is treated incorrectly, as a result wrong date value is converted to the action. These are examples of correct date formats:
<sj:datepicker value="today" id="date3" name="date3" displayFormat="dd.mm.yy" label="Today" />
<sj:datepicker value="yesterday" id="date4" name="date4" displayFormat="mm/dd/yy" label="Yesterday" />
<sj:datepicker value="tomorrow" id="date5" name="date5" displayFormat="DD, d MM yy" label="Tomorrow" />
<sj:datepicker value="2004-08-14" id="date6" name="date6" displayFormat="d M, yy" label="String Value" />