Thanks for the response Shawn.
The relative dates option does not work for dashboard actions used as filter.
It only works for regular fields when used as filter.
I did not see the reference to 'actions' in the body of your original post, and obviously missed it in your title. If you post a packaged workbook with some sample data, I (we) won't have to guess at your situation. Thanks,
PS: You can't write to or 'update' a parameter. This is the #1 feature request in the Ideas section. You can however have a Date Parameter that allows your user to select dates, and then you can run calculations/actions off of this.