A little doubt, assignment this.pause_time(line 178) repeated, because next mothod this.stop() will do it.

[LianJilu]

Tock.prototype.pause = function () {
    if ( this.go ) {
      this.pause_time = this.lap();
      this.stop();
    }
   ...
 };

Tock.prototype.stop = function () {
   this.pause_time = this.lap();
   this.go = false;

   clearTimeout(this.timeout);
   ...
};