Stop Watch In Javascript

<SCRIPT LANGUAGE="JavaScript">
/*
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*/

var ms = 0;
var state = 0;

function startstop() {
if (state == 0) {
state = 1;
then = new Date();
then.setTime(then.getTime() - ms);
} else {
state = 0;
now = new Date();
ms = now.getTime() - then.getTime();
document.stpw.time.value = ms;
  }
}

function swreset() {
state = 0;
ms = 0;
document.stpw.time.value = ms;
}

function display() {
setTimeout("display();", 50);
if (state == 1)  {now = new Date();
ms = now.getTime() - then.getTime();
document.stpw.time.value = ms;
  }
}

window.onload=display
</SCRIPT>


<CENTER>
<FORM NAME="stpw">
Time:
<INPUT TYPE="text" Name="time">
<INPUT TYPE="BUTTON" Name="ssbutton" VALUE="Start/Stop" onClick="startstop()">
<INPUT TYPE="BUTTON" NAME="reset" VALUE="Reset" onClick="swreset()">
</FORM>
</CENTER>