Hi Paul,
You can create a formula like:
ToText ({?StartDateParam}, "MM/yyyy");
This will return a string like "06/2014".
You can also parse the date parts out of the parameters and format the display how you want:
NumberVar myYear;
NumberVar myMonth;
NumberVar myDay;
myYear := Year ({?StartDateParam});
myMonth := Month ({?StartDateParam});
myDay := Day ({?StartDateParam});
ToText (myMonth, 0, "", "") & "/" & ToText (myYear, 0, "", "");
This gives you the same results as the first one.
Hope this helps,
Brian