> Sure, it all seems to work for now... and then a customer signs up from Tuvalu, and performs a billable action at 11:59PM on the last day of the month, and yo

> Sure, it all seems to work for now... and then a customer signs up from Tuvalu, and performs a billable action at 11:59PM on the last day of the month, and your whole billing system falls over because they now have a split invoice generated for next month while it's still this month. A one-off is not "whole billing system falls over". These things are inevitable because bug free software never happens, and so humans exist to deal with these situations. Unlike what an update that changes API would do. Now you can't update unless you rewrite everything, and you certainly will have regressions just because of this rewrite