When adding or subtracting months, if the resulting time is a date
that does not exist, the result of this operation will always be the
last day of the intended month.
### Example:
(new Chronos('2015-03-01'))->subMonths(1); // Results in 2015-02-01
(new Chronos('2015-03-31'))->subMonths(1); // Results in 2015-02-28