Date conversion issue.

Hi.

I have seperate sheets for each month of the year and now i want to merge all the sheets into 1 large sheet.
But my date column in each sheet only had a number denoting the day of the month.

I have formatted the date column to dd-mmm, but obviously excel does not know what month it is so it defaults the values to Jan.
How do i first convert the date in each sheet to its relevant month without having to rewrite each date ?

Thanks
Iqbal