src/Pure/Tools/build.scala

 
       val sessions = getopts(args)
 
       val progress = new Console_Progress(verbose = verbose)
 
-      val start_date = Date.now()
-
       progress.echo(
-        "Started at " + Build_Log.print_date(start_date) +
+        "Started at " + Build_Log.print_date(progress.start) +
           " (" + Isabelle_System.ml_identifier() + " on " + hostname(options) +")",
         verbose = true)
       progress.echo(Build_Log.Settings.show() + "\n", verbose = true)
 
       val results =

             soft_build = soft_build,
             export_files = export_files,
             build_hosts = build_hosts.toList)
         }
       val stop_date = Date.now()
-      val elapsed_time = stop_date.time - start_date.time
+      val elapsed_time = stop_date.time - progress.start.time
 
       progress.echo("\nFinished at " + Build_Log.print_date(stop_date), verbose = true)
 
       val total_timing =
         results.sessions.iterator.map(a => results(a).timing).foldLeft(Timing.zero)(_ + _).