<script type="text/javascript">

function onSelect(calendar, date) {
    var input_field = document.getElementById("idate");
    var y = calendar.date.getFullYear();
    var m = calendar.date.getMonth();     // integer, 0..11
    var d = calendar.date.getDate();      // integer, 1..31
    input_field.value = d+"/"+m+"/"+y;
     if  (calendar.dateClicked) {
    calendar.callCloseHandler(); // this calls "onClose" (see above)
    }
   };

  Calendar.setup(
    {
      inputField   : "idate",
      ifFormat       :    "%d/%m/%Y",     // format of the input field (even if hidden, this format will be honored)
      daFormat       :    "%A, %B %d, %Y",// format of the displayed date
      align          :    "Tl",           // alignment (defaults to "Bl")
      flat         : "calendar-container", // ID of the parent element
      flatCallback : onSelect,       // our callback function
<%SimpleDateFormat sdf =  new  SimpleDateFormat("dd.MM.yyyy hh24:mm:ss");%>
//ставишь нужный тебе формат
      date         : <%=sdf.format( new  Date())%>
    }
  );
</script>