First you have to define what you mean by "one month ago". Is it 30 days ago? Or last month? Or a certain number of milliseconds ago?

No library can help you with that. You have to define what you actually want to calculate.

At some point you will need to calculate how many leap years to account for and whether or not a leap second was introduced in between two arbitrary times.

Time libraries can help you with that.

Exactly there is the same problem, to a lesser extend, when adding/subtracting years which is not trivial to do properly.

Let alone determining requirements for thresholds.

This discussion reminds me of this bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1715455

Why can’t you use Date.getFullYear with setFullYear?

Absolutely. moment has a definition of that and compute it in a consistent way, which is useful. But if you want to come up with a different definition then it cannot help you indeed.

>What is one month ago when you are March, 31st?

"one month ago" is between "a few weeks ago" and "a few months ago".

>We have to deal with monthly subscriptions and it's not fun.

it is a lot less unfun if you define a month as N calendar days in your terms of service, like virtually everyone does

Yes, that's what we do mostly, except to track external subscriptions that actually use months (I believe the Google playstore does that but I could be mistaken with another service)

And they would be guilty of misleading the consumer when doing so, and possibly accounting fraud as well because neither GAAP nor IFRS recognise such a calendar; the closest available is the 4-4-5 standard.

This is why we don’t let engineers write contracts. Many seem to think that law is a programming language in which they can redefine reality. It ain’t.