IF statements with multiple dates

tmgraham ✭✭
edited 12/09/19 in Smartsheet Basics

I am trying to have multiple columns with multiple dates automatically update my "next due date" column. For example I have 4 columns with 4 different dates, I want my due date column to be the last date + 90 days. 

For example if I have columns, A,B,C,D, and E, and all of those columns have a date except E, I would want my due date to go off of Column D + 90 days, but then when I do input a date into column E I would want my due date column to automatically update + 90 days from that date.

Any help that can be offered would be appreciated.