jQuery reset setInterval timer

My Jquery:

function myTimer() {
    var sec = 15
    var timer = setInterval(function() { 
    $('#timer').text(sec--);
    if (sec == -1) {
      clearInterval(timer);
      alert('done');
     } 
    }   , 1000);

}

$("#knap").click(function() {
    myTimer();
});

$("#reset").click(function() {
// set timer to 15 sec again.. 
});

I want the timer to be reset when clicked on #reset.