Hi Jeet

The first thing to clarify here is that the aggregate/grouping column option in report builder does not change the display format for the date time. It is used to control how the timestamp is transformed before it is aggregated. This is required because timestamps are down to the second, and that is of no use when aggregated, you will want it per day, or per month etc.

The actual display of the column is the nice_date display function, and that uses the strfdateshortmonth string from each language pack's langconfig.php.

Changing this setting may give you the behaviour you want but should be done with caution. This string is used through the entire site, not just for the date column in reports. Changing this will have an effect throughout the site (not just in reports).