fix exchange_sig: correctly group and order denominations

src/time.mli

@@ -38,6 +38,8 @@ module Timestamp : sig
   val never : t
   val zero : t
   val compare : t -> t -> int
+  val min : t -> t -> t
+  val max : t -> t -> t
   val diff : t -> t -> Relative.t
   val of_s : int64 -> t