diff --git a/app/models/user.rb b/app/models/user.rb
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -192,7 +192,7 @@
 
   def update_start_time
     stat = self.contest_stat
-    if stat == nil or stat.started_at == nil
+    if stat.nil? or stat.started_at.nil?
       stat ||= UserContestStat.new(:user => self)
       stat.started_at = Time.now.gmtime
       stat.save
@@ -260,6 +260,10 @@
     end
   end
 
+  def self.clear_last_login
+    User.update_all(:last_ip => nil)
+  end
+
   protected
     def encrypt_new_password
       return if password.blank?