When you say you want to create a report on down alerts do you mean nodes down? In which case create a report using the following criteria:
Then create a table which includes event time field, node name and whatever else you want to show:
You can now restrict to dates either in the table view:
or if you submit, now that you have a date/time field you'll see an additional dropdown on the report front page to restrict the data returned from the DB:
I'm guessing the best for resources would be to restrict via the datasource rather than filtering on the table.