USe either of these two options.
Enclose the expression in the highlighted step with:
GetAsDate ( //Your expression here )
Or use the date function:
Date ( Doner Detail::Standing Order Day of the Month ; Month (Main::eventDate ) ; Year ( Main::event_Date ) )
Dates are stored as integers that count the number of days since 12/31/0000 not as text so you have to convert the text form of your date into that number.