set excel column format while exporting

When I export datagrid data to excel using above code everything works fine but here i want to set format of one of the columns before export.
for eg:- one of the columns has date in all the cells due to which all the cells format is automatically converted from general to date format, i want to avoid this format conversion or even setting the format forcefully will do.
if any ideas, please reply ASAP.
Thanx in advance.