--- time_entry_orig.rb	2013-11-14 15:25:09.195440648 +0200
+++ time_entry.rb	2013-11-14 15:25:27.135231459 +0200
@@ -85,6 +85,7 @@
     errors.add :hours, :invalid if hours && (hours < 0 || hours >= 1000)
     errors.add :project_id, :invalid if project.nil?
     errors.add :issue_id, :invalid if (issue_id && !issue) || (issue && project!=issue.project)
+    errors.add :spent_on,  "is too early" if (spent_on < Date.today - 5.day)
   end
 
   def hours=(h)