Radio button properties field type is greyed out - can't change it

I am trying to add a reset form button to my PDF form.  I added a check box and when I tried to change the properties: field type, it was greyed out and defaulted to a field type of Date.  So I went back and looked at all of my other buttons on the form, both check box and radio buttons and they are all defaulted to a Field Type of Date and all greyed out and I can't change them.  Any ideas?