Comment by josephg

> It would have been nice for other wide changes like this to have like a 'strict datetime'; directive which would opt you into using this corrected behavior.

That would be ugly, because you'd want some parts of your program (eg libraries) to use the old behaviour, and other parts might want the new behaviour. How would you minify multiple modules together if they all expect different behaviour from the standard library?

In my opinion the right way to do this is to have multiple constructors (as Obj-C, swift, C and rust all do). Eg:

    let d = new Date(...) // old behaviour (not recommended for new code)
    
    let d = Date.fromISOString(...) // fixed behaviour

The big downside of this is that its tricky to keep track of which fields and functions people should really stop using in modern javascript. It'd be nice if there was a way to enable more shouty warnings during development for deprecated JS features.