diff --git a/app/models/diploma.rb b/app/models/diploma.rb
index a5519f6..3950053 100644
--- a/app/models/diploma.rb
+++ b/app/models/diploma.rb
@@ -25,7 +25,7 @@ class Diploma
   before_validation :add_http
 
   def duration
-    self.start_date.to_s+' ~ '+self.end_date.to_s
+    self.start_date.strftime('%Y.%m')+' ~ '+self.end_date.strftime('%Y.%m')
   end
 
   def slug_title