Datepicker

Hi all,
I want to call my function when after select the date. For example,
using calender.setup I have used the datepicker. If i select the wrong date within textbox, without move out from the box I want to show some alert. How can i do this.
For example steps,
textbox - select date - call the alert
Whether is any possiblity like,

Calendar.setup({ inputField : 'start_date',
ifFormat : 'dd/mm/y',
singleClick: 'true'
validate : check_current_date()[/u][/b]
});