The expected format of an EDM DateTime is the milliseconds: http://www.odata.org/documentation/odata-version-2-0/json-format/
So the workaround to see a string form would be to add a view or modify the existing table to expose the timestamp column as a string type instead.
Thanks for the reply.
On changing the datatype to String, i am getting an error - "The column 'CREATION_DATE' with runtime type 'string' does not match the runtime type 'timestamp' from the query transformation".
I had only changed the data type in view table. On changing in Source table also i am getting the expected output.
You can also update the view select clause and cast to string - cast(timstampval as string)