@Test
public void invokePeriodMonths() {

    FunctionTestUtil.assertResult(stringFunction.invoke(Period.ofMonths(2)), "P2M");
    FunctionTestUtil.assertResult(stringFunction.invoke(Period.ofMonths(27)), "P2Y3M");
    FunctionTestUtil.assertResult(stringFunction.invoke(Period.ofMonths(-27)), "-P2Y3M");
}