Rollback 28 days expression

[Anonymous]

I use the following expression to calculate an average rolling back 28 days

((Sum({<Promo ={0},DateTest={">=$(=Date(Today() - 30)) <$(=Date(Today()-2))"}>} [Prix de vente TTC]))/28 )

how to make a rollback in case Promo ={0} for 28 days ? How to make a rolback for the 28 fisrt days back where Promo ={0} ?

[Chanty4u]

=if(agg(Sum({<Promo ={0},DateTest={">=$(=Date(Today() - 30)) <$(=Date(Today()-2))"}>} [Prix de vente TTC]))/28 ))