code: function(microseconds) {
          var milliseconds = microseconds / 1000;
          if (moment.utc(milliseconds).format("HH") > 0) {
              return moment.utc(milliseconds).format("HH:mm:ss.SSS");
          }
          return moment.utc(milliseconds).format("mm:ss.SSS");
      }
docs: formats microseconds into VTT timestamps
  @function
  @param {string} microseconds - microseocnds to convert to VTT timestamp
  @public