See if this helps! I found this code online.
<HEAD>
<SCRIPT LANGUAGE="JavaScript">
var up,down;var min1,sec1;var cmin1,csec1,cmin2,csec2;
function Minutes(data) {
for(var i=0;i<data.length;i++)
if(data.substring(i,i+1)==":")
break;
return(data.substring(0,i));
}
function Seconds(data) {
for(var i=0;i<data.length;i++)
if(data.substring(i,i+1)==":")
break;
return(data.substring(i+1,data.length));
}
function Display(min,sec) {
var disp;
if(min<=9) disp=" 0";
else disp=" ";
disp+=min+":";
if(sec<=9) disp+="0"+sec;
else disp+=sec;
return(disp);
}
function Up() {
cmin1=0;
csec1=0;
min1=0+Minutes(document.sw.beg1.value);
sec1=0+Seconds(document.sw.beg1.value);
UpRepeat();
}
function UpRepeat() {
csec1++;
if(csec1==60) {
csec1=0; cmin1++;
}
document.sw.disp1.value=Display(cmin1,csec1);
if((cmin1==min1)&&(csec1==sec1))
alert("Stopwatch Stopped");
else up=setTimeout("UpRepeat()",1000);
}
</SCRIPT>
<BODY>
<CENTER>
<FORM name="sw">
<TABLE border="0" width="100%">
<tr align="center">
<td><table border="3" width="100%"><tr>
<th colspan="2">Stopwatch</th></tr>
<tr align="center">
<td>Stop at<br>
<input type="text" name="beg1" size="7" value="0:10"></td>
<td><input type="button" value="Start" onclick="Up()"></td>
</tr>
<tr align="center"><td colspan="2">
<input type="text" name="disp1" size="7"></td></tr></table></td>
</TABLE>
</FORM>
</CENTER>