add timestamp.ml

WIP better time
  revert this?

src/signkey.ml

@@ -5,9 +5,9 @@ open Crypto
 type t = {
   pub: EddsaPublicKey.t;
   priv: EddsaPrivateKey.t;
-  stamp_start: Ptime.t;
-  stamp_expire: Ptime.t;
-  stamp_end: Ptime.t;
+  stamp_start: Timestamp.t;
+  stamp_expire: Timestamp.t;
+  stamp_end: Timestamp.t;
   (* signature of this key by offline master key *)
   master_sig: EddsaSignature.t option;
 }
@@ -17,13 +17,13 @@ let generate () =
        - look if it exists
        - if not, create it (TOFU initialization scheme)
        - write it *)
-  (* TODO time
-     correctly set those: *)
-  let stamp_start = Ptime_clock.now () in
+  let stamp_start = Ptime_clock.now () |> Option.some in
   let stamp_expire =
-    Util.ptime_add_span_exn stamp_start Config.Exchange.signkey_legal_duration
+    Timestamp.add_span_exn stamp_start
+      (Some Config.Exchange.signkey_legal_duration)
   in
   let stamp_end = stamp_expire in
+
   let priv, pub = Mirage_crypto_ec.Ed25519.generate () in
   let master_sig = None in
   { pub; priv; stamp_start; stamp_expire; stamp_end; master_sig }