When using External<...> the ... ends up in the index.d.ts as an unknown type

[CMCDragonkai]

I'm using the quiche library and I'm exposing:

#[napi(object)]
pub struct SendInfo {
  /// The local address the packet should be sent from.
  pub from: Host,
  /// The remote address the packet should be sent to.
  pub to: Host,
  /// The time to send the packet out for pacing.
  pub at: External<std::time::Instant>,
}

When this generates a index.d.ts I end up with:

export interface SendInfo {
  /** The local address the packet should be sent from. */
  from: Host
  /** The remote address the packet should be sent to. */
  to: Host
  /** The time to send the packet out for pacing. */
  at: ExternalObject<Instant>
}

However the Instant is an unknown type. There's no such equivalent JS type for it.

This creates an error in my IDE (and maybe later during tsc compilation).

Shouldn't the TS version of the type just be some opaque object type, why is it exposing the Instant?

How should I fix this?