you're are dealing with 2 different types, so u have 2 possibilities, cast the
string to a date, or cast the date to a string,
I would try the other one: the string to a date, cos you could have some
different possibilities...

(DateTime) this.DIA_RETIRADAT.Text =
this.objREserva.agretirada.dtRetirada.("dd")

=> possible you have to write smth as system.data.sqlclient.datetime