How to pop-up dialog box when user selects a date which is less than the current date


I am having one requirement.

<input type=“text” name=“effectivedate” id=“effectivedate” value=“2010-03-16” dojoType=“dijit.form.DateTextBox” required=“true” promptMessage=“Enter service date.” invalidMessage=“Invalid date.” constraints=“{min:(new Date(1901, 01 , 01)), max:(new Date(2099, 12, 31)), datePattern:‘dd MMM yyyy’}” />

here i am storing the default value has, value=“2010-03-16”.
So if user selects any date other than this, he has to get a pop-up dialog box saying (Are you sure to select the selected date) if user clicks YES button in the dialog box then the selected date has to store in the box. if he choose the button NO then the default value has to store(value=“2010-03-16”).

Please help me on this.