You'll need to include a date calculation in your query to produce the minimum starting date, and then concatenate that into your query, like this:
concat('[abbreviated...]<gt><column name"StartDate" /><value>', xdDate:AddDays(xdDate:Today(), -30), "</value></gt>[abbreviated...]")
I have a feeling that query builder will not allow you to do this, but you can produce the bulk of your query in QueryBuilder, inserting a dummy value for the StartDate part, and then manually replace the dummy value with the xdDate:AddDays(xdDate:Today(), -30) formula.