code: function (dt, startOfWeek) {
          startOfWeek = startOfWeek || 0;
          var dayOfWeekIndex = dt.getDay(),
              dayOfWeek = (dayOfWeekIndex - startOfWeek + 7) % 7;
  
          return this.subtract(dt, this.DAY, dayOfWeek);
      }
docs: Get the first day of the week, for the give date.
  @param {Date} dt The date in the week for which the first day is required.
  @param {Number} startOfWeek The index for the first day of the week, 0 = Sun, 1 = Mon ... 6 = Sat (defaults to 0)
  @return {Date} The first day of the week