Calculate last 6 months

for a report parameter for the end of the report you could use this as the parameter default value to get the last day of prev month

=cdate(Format(dateadd("d",(day(now()))*-1,now()),"dd/MMM/yyyy"))

To get the start date, 6 months prior to today, use the function supplied by Kev

...
where datecreated > dateadd(mm, -6, getdate())
...

or do you need a better definition of 'month'?