Greetings Prasad,
I'm a novice with Excel, but I may have a very simple solution for you to try.
For this example, I’m assuming the dates you listed in your example (such as 24/09/2012) are at cell reference A1 and continue down that column, and I’m inserting my formula in a column next to it. Just adjust the “A1” reference as needed to fit your spreadsheet and copy the formula down as needed.
The basic problem seems to be just reversing the day and month entries, and keeping the year in place, so I just pulled out the LEFT, MID and RIGHT commands and juggled their placement to match your requirements. I had to add the VALUE function to encompass the entire formula to make the dates actually work as a date.
=VALUE(MID(A1,4,2)&"/"&LEFT(A1,2)&"/"&RIGHT(A1,4))
This isn’t an actual date, just a formula that “looks” like a date – it’s just the results of the formula. If you actually want to go a step further you can highlight the dates with my formula, copy the values, and using the down arrow under the PASTE button in the Excel Ribbon, select “Paste Values” (keyboard shortcut ALT, H, V, V). This will replace the formula in the cell with the actual value. If the cell is formatted as “General”, the 9/24/2012 date will appear as “41176”, which is how Excel represents dates. If you change the cell formatting to short date, long date etc., you would end up with (9/24/2012) and Monday, (September 24, 2012) respectively.
I hope this works...and as I said, I'm a novice, so I'm just taking a shot.