OnBlur/OnChange event to validate date

I was using the following answer to help me:
http://www.experts-exchange.com/Web/Web_Languages/JavaScript/Q_11478358.html?query=on+blur+date+valid&topics=87

but, it seems left off where I most need it.  "Bushweiler" explained that there is a way to not let the user leave a text box where there is invalid data.  I can't seem to figure out how to do this.  Please help!


I am trying to set the focus back to the box with the error.  

<input tabindex="12" type="Text" size="10" name="txtDate3" onChange="CheckDate(this,'', this.form.name, this.name);">

This is my checkDate function:
/**
 * DHTML date validation script. Courtesy of SmartWebby.com (http://www.smartwebby.com/dhtml/)
 */
// Declaring valid date character, minimum year and maximum year
var dtCh= "/";
var minYear=1900;
var maxYear=2100;

function isInteger(s){
      var i;
    for (i = 0; i < s.length; i++){  
        // Check that current character is number.
        var c = s.charAt(i);
        if (((c < "0") || (c > "9"))) return false;
    }
    // All characters are numbers.
    return true;
}

function stripCharsInBag(s, bag){
      var i;
    var returnString = "";
    // Search through string's characters one by one.
    // If character is not in bag, append to returnString.
    for (i = 0; i < s.length; i++){  
        var c = s.charAt(i);
        if (bag.indexOf(c) == -1) returnString += c;
    }
    return returnString;
}

function daysInFebruary (year){
      // February has 29 days in any year evenly divisible by four,
    // EXCEPT for centurial years which are not also divisible by 400.
    return (((year % 4 == 0) && ( (!(year % 100 == 0)) || (year % 400 == 0))) ? 29 : 28 );
}
function DaysArray(n) {
      for (var i = 1; i <= n; i++) {
            this[i] = 31
            if (i==4 || i==6 || i==9 || i==11) {this[i] = 30}
            if (i==2) {this[i] = 29}
   }
   return this
}

function isDate(dtStr){
      var daysInMonth = DaysArray(12)
      var pos1=dtStr.indexOf(dtCh)
      var pos2=dtStr.indexOf(dtCh,pos1+1)
      var strMonth=dtStr.substring(0,pos1)
      var strDay=dtStr.substring(pos1+1,pos2)
      var strYear=dtStr.substring(pos2+1)
      strYr=strYear
      if (strDay.charAt(0)=="0" && strDay.length>1) strDay=strDay.substring(1)
      if (strMonth.charAt(0)=="0" && strMonth.length>1) strMonth=strMonth.substring(1)
      for (var i = 1; i <= 3; i++) {
            if (strYr.charAt(0)=="0" && strYr.length>1) strYr=strYr.substring(1)
      }
      month=parseInt(strMonth)
      day=parseInt(strDay)
      year=parseInt(strYr)
      if (pos1==-1 || pos2==-1){
            alert("The date format should be : mm/dd/yyyy")
            return false
      }
      if (strMonth.length<1 || month<1 || month>12){
            alert("Please enter a valid month")
            return false
      }
      if (strDay.length<1 || day<1 || day>31 || (month==2 && day>daysInFebruary(year)) || day > daysInMonth[month]){
            alert("Please enter a valid day")
            return false
      }
      if (strYear.length != 4 || year==0 || year<minYear || year>maxYear){
            alert("Please enter a valid 4 digit year between "+minYear+" and "+maxYear)
            return false
      }
      if (dtStr.indexOf(dtCh,pos2+1)!=-1 || isInteger(stripCharsInBag(dtStr, dtCh))==false){
            alert("Please enter a valid date")
            return false
      }
return true
}

function ValidateForm(){
      var dt=document.frmSample.txtDate
      if (isDate(dt.value)==false){
            dt.focus()
            return false
      }
    return true
 }



function CheckDate(entered, alertbox, form, field)
{
            with (entered)
                  {
          //if (isDate(entered)==false) why are we using value
            
            if (value=='N/A' || value == 'UNKNOWN' || value=='n/a' || value =='unknown' || value=='Unknown'){return true}
            else {if (value==null || value=="" || isDate(value)==false) {
            alert("You must fill in either a date or UNKNOWN")
            eval("document." + form + "." + field + ".focus()");
           }
            else {return true};};
            
            //if (value==null || value=="" || value!='N/A' || isDate(value)==false)
            //{if (alertbox!="") {alert(alertbox);} return false;}
            //else {return true;}
}
}
//            dt.focus()
//            return false
//            }
//    return true
//}