Specified cast is not valid in Linq?

Hi,

these is Database Structure

TimeSheetId    int               Unchecked
ReserveID        int               Unchecked
LoginTime        datetime      Checked
LogoutTime      datetime     Checked
Day                                     Checked
Hour                                   Unchecked
       

first i bind normally , its work properlly

but i formulate Hour Field like

(CONVERT([nvarchar](52),[LogoutTime]-[LoginTime],(108)))

its giving a error :

Specified cast is not valid.]
while i refresh the table from server explorer and again drag and drop it
thanks