Martin,

There is not a way to do what you want in SSRS 2000 or 2005 (not sure on 2008). You'll need to create a second version of your report with the date formula/expression as a default parameter value. Use that version for your subscription and the original for on demand reporting.

Sometimes it's easier to create a SQL stored proc that generates the desired date and reference that as your default. Added benefit of this is you can reuse the SP whenever you need this type of parameter value in future reports.