If you haven't already gotten an answer, here is an easy way to do it:
Drag a Block onto your report in the Heading area.
Drag a Layout Calculation into the Block.
On the resulting Report Expression window, click the "Functions" icon, which will drop down several standard value functions.
Double-click the "AsOfDate" function. The function will show in the window work area.
Click "OK" at the bottom of the window.
Run the report. You should have your run date displayed in the heading.