You can't change the column datatype if a
DataTable
has data in it.
You can try making sure the column is set to be a date/time in excel before reading it.
I wrote an article that includes code which allows you to import xlsx or csv files, and if you want/need to, you can manually specify the datatype for each column, or let the code finger it out on its own.
CSV/Excel File Parser - A Revisit[
^]