calendar: migrate extension for DateTime

web/ASC.Web.Core/Calendars/RecurrenceRule.cs

@@ -73,6 +73,12 @@ namespace ASC.Web.Core.Calendars
             }
             return count;
         }
+
+        public static string ToShortString(this DateTime targetDateTime)
+        {
+            return String.Format("{0} {1}", targetDateTime.ToShortDateString(), targetDateTime.ToShortTimeString());
+        }
+
     }
 
     public enum Frequency