How to add 30 days to existing date

Question

If a member signs into my site then on sql it inserts the members name , password , date as now().
I created another item on my table called datetr. I want it must calculate +30days from the actual date they sign in (now()) and update the datetr.

Basically I am giving them a 30 day trial and then they need to be blocked if they have not paid.
Maybe you have a better idea to do this.