Get function has specific parameters you can use. They are listed in FileMaker help and "givendate" isn't one of them.
Your calculation could be corrected by doing this:
Let ( D = givendate ; Date ( Day ( D ) - 100 ; Month ( D ) ; Year ( D ) ) )
But you can do this much simpler by simply using this expression:
givendate - 100
Note: in both of these examples, I am assuming that you are defining either a a calculation field with Date selected as the result type or a date field with an auto-enter calculation and that this new field is defined in the same table as your field named "givendate".
Thanks a lot, that fixed it for me!