How to exclude incorrect or inaccurate log files???

I am using the Professional version. Occasionally (once or twice a year) my hosting company (or their server) fails to create a log (or generates an abbreviated log ) on a particular day. I maintain reports by month and would like to know if there is a way to exclude a specific date from the date range for analysis. Now I set a range, say July 1 - July 30. If the log for July 21 is messed up (no or few log entries) then Weblog makes it's report for the full month including July 21 which skews the real summary values for visitors, page views, etc. downward. I need to tell Weblog analyze the logs from July 1 to July 30, except exclude the log from July 21, and make the summaries correct for 29 days (not 30). I tried just not including the inaccurate log file, but Weblog makes it's calculations based on the full date range (i.e. 30 days). I hope explained the problem clearly enough.