Sounds like we need an extended JSON with the express intent of conveying common extended values and rich objects: DateTime instants (with calendar system & timezone), Decimal, BigInt, etc.
replyI disagree: this is not unlike including the schema in the JSON itself. This should be handled by the apps themselves, since they would have to know what the keys mean regardless.
replyIf you do want the interchange format to be the one deserializing into specific runtime data structures, use YAML. YAML's tag syntax allows you to run arbitrary code inside YAML, which can be used for what you want.
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.
replyThe 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.