testOffsetInMonths() {
    const d = new DateDate(2008, 9, 13);
    /** @suppress {visibility} suppression added to enable type checking */
    const f = DateRange.offsetInMonths_;

    assertTrue('this month', new DateDate(2008, 9, 1).equals(f(d, 0)));
    assertTrue('last month', new DateDate(2008, 8, 1).equals(f(d, -1)));
    assertTrue('next month', new DateDate(2008, 10, 1).equals(f(d, 1)));
    assertTrue('next year', new DateDate(2009, 9, 1).equals(f(d, 12)));
    assertTrue('last year', new DateDate(2007, 9, 1).equals(f(d, -12)));
  }
@suppress {visibility} suppression added to enable type checking