จะหาค่าผลเทียบ เวลา ของ text box 2 ช่อง โดยแสดงผลเลยที่กรอกสองช่องดังกล่าว มีวิธีไหมครับ

<script language="javascript">
                          function fnccheck(){
                            var t1;
                            var t2; 
                            var sum;
                              t1=parseFloat(document.frm.text1.value);
                              t2=parseFloat(document.frm.text2.value);
                            
                            sum=t2-t1;
                            document.frm.total.value=sum;
                          }

<input type="text" id="txt1">
<input type="text" id="txt2">
<input type="text" id="txt3">

<script>
function GetTimeDiff() {

    var txt1 = document.getElementById("txt1").value;
    var txt2 = document.getElementById("txt2").value;

    start = txt1.split(":");
    end = txt2.split(":");
    var startDate = new Date(0, 0, 0, start[0], start[1], 0);
    var endDate = new Date(0, 0, 0, end[0], end[1], 0);
    var diff = endDate.getTime() - startDate.getTime();
    var hours = Math.floor(diff / 1000 / 60 / 60);
    diff -= hours * 1000 * 60 * 60;
    var minutes = Math.floor(diff / 1000 / 60);
    
    var result = (hours < 9 ? "0" : "") + Math.abs(hours) + ":" + (minutes < 9 ? "0" : "") + minutes;
    document.getElementById("txt3").value = result;

}

document.getElementById("txt1").onchange = function(){ 
     GetTimeDiff();
};
document.getElementById("txt2").onchange = function(){ 
     GetTimeDiff();
};
</script>