Dates displayed as Number : POI API's-POI Horrors !!!

[Birendar Waldiya]

Hi  All, 
 
I am facing an unpredictable behaviour of excel date while reading excel
files using POI, sometime the dates appear as dates  and somtine they appear
as number , 
this often happens when I first chose a date format say mm/dd/yyyy read the
excel. Now I chage the date format dd/mm/yyyy now the date start appearing
as number No matter what format I go now. 
Can anyone tell me why this is happening?
 
Thanks 
Regards 
Birendar S Waldiya