This is a legitimate warning depending on whether you are scheduling a single run report or a recurring report.
For a single run (e.g., an after-hours report), having a non-dynamic date is fine, and you can continue without a problem. For a recurring report (e.g., every month end), typically you should use a dynamic date. Otherwise, the report will run with the same date parameters every time.
For example, if I want a report to run every week, and I specify an Invoice date range of 04/20/13 to 05/05/13, I would get the exact same selection each week, since I've specified a non-dynamic date. If I just want to schedule the report for a one-time run at 2am on 05/06/13 (so it's done before I get to work next Monday morning), having a non-dynamic date is fine.
On the other hand, if I want the report to run every Monday at 2am, I would specify the date range as -7 to -1. This would use the dates seven days prior to one day prior to the date the report runs.
Remember, you can use a signed integer to specify a date relative to the current date (e.g., +14 is 2 weeks from today), or T to specify the current date. A recurring scheduled report typically needs dynamic dates of this sort.
