Test framework - API4 DateTest::testRelativeDateRanges will fail on the last day of Jan, Mar, May, Aug, Oct
And it might fail other times.
The test compares for example strtotime('+1 month') to what is ultimately the calculation from CRM_Utils_Date::relativeToAbsolute() which has a different definition of e.g. 'next.month'. So e.g. on Mar 31 the strtotime() will return May 1 not Apr 31 (which doesn't exist), but the getFromTo() will return Apr 1 (always the first of "next month").