How filter grid data based on additional search field

Hello,
I got a grid with a search on top . Only on search fields I added a field i.e. (due_days_to_add ) as integer .
How can use it correctly to make the search based on a field date + that integer value ? For instance {due_date} = date(“Y/m/d”) + {due_days_to_add}.

I did this code below within “Search” --> Events --> onFilterValidate but nothing happens… Is there a more brilliant solution ?

if (isset({due_days_to_add}))
{
sc_select_where(add) = "where {due_date} = " . date(“Y/m/d”) + {due_days_to_add};
}

How can I understand that the field i.e. {due_date} is that one on Search area and not that one of grid ?
I’m confused…

search.png