I have a problem how do i write back the c_file.time_write to the files ?

Code:
 if( (hFile = _findfirst( "*.bmp", &c_file )) == -1L )
       printf( "No *. bmp files in current directory!\n" );
   else
   {

           c_file.time_write = rand(lDateTime);
   }

while( _findnext( hFile, &c_file ) == 0 )
            {
                  c_file.time_write = rand(lDateTime);
             }

Basically i need to change the dates to random values if there is a way please tell me anyone