Haos616

<input type="text" id="my_timer" value="5"> сек.

<button type="submit" class="sbmt" onClick="startTimer()">Запуск с таймером</button>

<script type="text/javascript">
window.startTimer = function() {
     var my_timer = document.getElementById("my_timer");
     var time = my_timer.value;
   
     var s = my_timer.value;
	
    if (s == 0) {
			alert('Tst1',1);
          return;
      
    }
    else s--;
    if (s < 10) s = "0" + s;
    document.getElementById("my_timer").value = +s;
    setTimeout(startTimer, 1000);
}
</script>