시간 차이를 실시간으로 표여 주고 싶습니다.

<script>
function now_time() {
    var time_t = new Date();
    var s =
    set_standard(time_t .getHours(), 2)+':'+
    set_standard(time_t .getMinutes(), 2);
    document.getElementById("output1").value = s;
}

function now_time2() {
    var time_t = new Date();
    var s =
    set_standard(time_t .getHours(), 2)+':'+
    set_standard(time_t .getMinutes(), 2);
    document.getElementById("output2").value = s;
}

function set_standard(time, digits) {
    var zero = '';
    time = time.toString();
    if (time.length < digits) {
        for (i = 0; i < digits - time.length; i++)
        zero += '0';
    }
    return zero + time;
}
</script>        

            
<td>
<input type="text" name="output1" id="output1" value="<?php echo $write['output1']; ?>">
<input type="button" onclick="now_time(this);"value="시작 시간">
</td>
<td>
<input type="text" name="output2" id="output2" value="<?php echo $write['output2']; ?>">
<input type="button" onclick="now_time2(this);" value="종료 시간">
</td>
<td>
<input type="text" name="output3" id="output3" value="<?php echo $write['output3']; ?>">
</td>