fix exchange_sig: correctly group and order denominations

src/time.ml

@@ -99,6 +99,8 @@ module Timestamp = struct
   let never = uint64_max
   let zero = 0L
   let compare = Int64.unsigned_compare
+  let min a b = if compare a b < 0 then a else b
+  let max a b = if compare a b > 0 then a else b
 
   (* zero if a >= b; never if b=never; otherwise b - a *)
   let diff a b =