The "Computers with approved update percentages" report in the Windows Server Update Services report category cannot currently be enhanced to filter on Release Date, even though that filter is available in the Updates datasource. The Report Builder returns a "multiple datasource" error, which it should not be doing for reports in this category.
However, a similar report can be obtained from the Windows Server Update Services Analytics report category by using the "Approved Summary with percentages" report, which already contains Release Date as a default field. You can dynamically filter this value from the Report Viewer by de-selecting the current month from the display, or you can save a report specification by adding a filter using Report Builder for Release Date "was not in this month". (Note that this value is based on Calendar Month, not "the last 30 days".)