It's not that much about type safety. Since TypeScript uses duck typing, a DateTime could not be used as a ZonedDateTime because it'd lack the "timezone" property. The other way around, though, it would work. But I wouldn't even mind that, honestly.
The real drawback of the functional approach is UX, because it's harder to code and you don't get nice auto-complete.
The real drawback of the functional approach is UX, because it's harder to code and you don't get nice auto-complete.
But I'd easily pay that price.