timer.js

function callTimer() {
  $('.counter').counterUp({
        delay: 20,
        time: 2000
  });
}

$(document).ready(function() {
  var target = $('#target');
  $(window).scroll(function() {
    var targetTop = target.position().top - 10;
    var winScrollTop = $(window).scrollTop();
    if(winScrollTop >= targetTop) {
       callTimer();
    }
  });
});