I am going to assume you have to have the T or Z in there for some other purpose (else you would have simply removed it already.)
I would either:
add another column to the excel file and replicate the date/time without the T or Z,
Do a calc field in Tableau that does string manipulation to replace "Z" with " " or replace "T" with " ". (Or some other appropriate treatment of the Ts and Zs.) Be sure to convert the manipulated string into data format.