<form name="formTime">
  <input type="text" name="clock" size="8">
</form>

<script>
function startClock()
{
  var dateNow = new Date()
  var hours = dateNow.getHours()
  var minutes = dateNow.getMinutes()
  var seconds = dateNow.getSeconds()
  var dn = ""
  if (hours < 10) hours = "0" + hours
  if (minutes < 10) minutes = "0" + minutes
  if (seconds < 10) seconds = "0" + seconds
  // formTime - имя формы с текстовым полем
  // clock - имя текстового поля
  document.forms['formTime'].clock.value = " " + hours + ":" + minutes + ":" + seconds + " " + dn
  setTimeout("startClock()", 1000)
}
startClock()
</Script>