Excel help - problem with date format

Hello all,

Actually I created a spreadsheet a while ago now I need to use that one and my problem is that when I entered data that time I used format dd/mm/yyyy like 13/01/99 but now if I am trying to change in 13-jan-99 ........... but up to up to 12 system converts date in to month and after it becomes changeless............

So please anyone can help me in this matter.........