Do you want to limit the imput of a text input (textbox) to valid date characters?  Here you go:

// START SCRIPT

function runKeyFilter(e) {
 if (e.srcElement.readOnly) return;

 var key_code = e.keyCode;
 var oElement = e.srcElement;
 //alert (key_code);

 try {
  if (!window.event.shiftKey && !window.event.ctrlKey && !window.event.altKey) {
   if (
     (key_code > 47 && key_code < 58) ||  // numbers on top of keyboard
     (key_code > 95 && key_code < 106)  ||  // number on numpad at right of keyboard
     (key_code > 36 && key_code < 41) ||  // arrow keys
     (key_code == 35) || // end
     (key_code == 36) || // home
     (key_code == 45) || // dashes
     (key_code == 111) || // forward slash /
     (key_code == 191) || // forward slash /                      
     (key_code == 8) || // backspace
     (key_code == 46) || // delete
     (key_code == 9) || // tab
     (key_code == 16) //SHIFT TAB
      )
   {
    e.returnValue = true;
    return;
   }      
  } 
 }
 catch (ex) {
  alert('Error Message: ' + ex.message );
 
 }
 // if we get here then we didn't have a number
 e.returnValue = false;

}  // end runKeyFilter()

//using the helper functions from prototype.js
Event.observe('|CONTROL|', 'keydown', runKeyFilter, false);  

// END SCRIPT