My point is more that if it costs more to fix it now, than it does to fix it later (including the opportunity cost), then there is little point in fixing an extreme edge case now. Especially if it means having to understand every intricacy of every library you work with and spend a long time thinking of permutations that have little effect on the business.

It depends on the situation you are in as to whether that cost calculation works out in your favour.

It is not always that is has to always work in 100% of edge cases, all thought about up-front and with no possible bugs arising due to not 100% understanding every line of code in every library. Sometimes it makes more sense to fix a bug that happens every four years when it causes a problem. In this case the bug may manifest itself in a user potentially seeing a day off by one error on a subscription details page. In which case it may never make sense to fix that page. As the user may never look at that page, and even if they did, they may never care.