Calculate date based on pattern from anchor date

Calculate date based on pattern from anchor date

I have a rolling 4 week (28 day) date pattern starting from an anchor date
in the past. I need to know what the nearest previous pattern start date
is relative to the current date.
For example:
Anchor date = Monday June 30 2013. If today's date is August 7 2013 then
the date I would like returned is July 29 2013. Previous pattern start
dates were June 30, July 29. The next pattern start date is August 26 but
that is after today's date of August 7 2013.
Hope that makes sense and thanks