Use ->addMonthsWithoutOverflow() and ->subMonthsWithoutOverflow()
Summary: Carbon::now()->subMonth(1) for 2020-03-30 will end up as 2020-03-01, which simply isn't a month.
Test Plan:
phpunit failed.
apply change.
phpunit succeeds.
Reviewers: #hkccp_developers, machniak
Reviewed By: #hkccp_developers, machniak
Subscribers: machniak, #hkccp_developers
Differential Revision: https://git.kolab.org/D1096