Or maybe extend this with types for the timestamp, `@ms:1623132000` or `@unix:1623132000`, so in a value: `{"now":@unix:1623132000}`

Mongo types the field name, `{"$date":1623132000}`, if I'm not mistaken. Rust style would be `1623132000_unix`.

Or with anticipation of a more full typing system, where the time is explicitly named:

`{"now": unix:1623132000}`

For now, when I need typing, I use https://json-schema.org