Dynamic decoders for opaque types

I'm having lots of fun learning gleam, by writing a TUI app to manage my reading lists and bookmarks. However I am a bit stuck while writing a decoder for my types:

pub type State {
  Valid(time: Timestamp, previous: State)
  Created(time: Timestamp)
  Invalid(time: Timestamp, previous: State)
}

pub type Link {
  Link(uri: Uri, title: String, description: String, state: State)
}

I think I got how to make a recursive decoder, but no idea how to decode Timestamp (from package gleam/time) because it is opaque.

My goal is to use a DETS for persistence (ideally using shelf) and I must provide decoders for keys and values.

What should I try ?