reuse timestamp function

--- a/src/HOL/Tools/Nitpick/kodkod.ML	Fri Apr 23 19:18:39 2010 +0200
+++ b/src/HOL/Tools/Nitpick/kodkod.ML	Fri Apr 23 19:26:39 2010 +0200
@@ -896,8 +896,7 @@
          out "solve "; out_outmost_f formula; out ";\n")
   in
     out ("// This file was generated by Isabelle (most likely Nitpick)\n" ^
-         "// " ^ Date.fmt "%Y-%m-%d %H:%M:%S"
-                          (Date.fromTimeLocal (Time.now ())) ^ "\n");
+         "// " ^ Sledgehammer_Util.timestamp () ^ "\n");
     map out_problem problems
   end