It sounds like your operator ID probably doesn't have a default locale set.
The date format is dependent in the first instance on the operator default locale set in your operator ID record. If this is not set then the format will depend on the regional settings of the machine you are logging in from.
Hi Marc, thanks a lot for the reply I got you point. Just one question that when you say "regional settings of the machine you are logging in from" how can I change the settings and test if the format change takes place or not.