HI,
what is the date format of your date object at webi level and it's string or date type.
if datatype is date then at webi level-right click on cell->Formatnumber->Date/Times->select the predefined format.if your specfic format is not in default one then. click custom and add Mmm dd,yyyy
if data type is string then first covert the object in date.
Var date=todate([your object];"dd/MM/yyyy") according to your date object format replace the same format instead of dd/MM/yyyy.
After that either right click->formatnumber steps as mentioned above.
or other way
=formatdate([Var date];"Mmm dd,yyyy")
Amit