in src/datetime/DateTime/DateTime.php [252:262]
final public function minusMonths(int $months): Builder<this> {
if ($months < 0) {
return $this->plusMonths(-$months);
}
$new_month_raw = $this->month - $months - 12;
return $this->withDate(
$this->year + (int)($new_month_raw / 12),
$new_month_raw % 12 + 12,
$this->day,
);
}