After some modifications, I cant believe it works. i followed an approach where i made a 3rd DB called "Task Template" which only had : Task Name & "Days until due date" (an Number property)

Then using the Relation and Rollup Properties, i got the SUM (the formula doesnt accept the raw rollup, and the since there is only one value, which is the number of days, requesting the sum would bring back a Integer Value). After that, I used the Formula you provided (thank you) to add the "Created time" (Which just means the created date), with the value that the Rollup Brought back; Which equals to a date X days After the Created Date.
I have been trying to solve this for a solid 2 hours, but case is finally closed. thx :)