How do I find a date after 3 months from today?

Question;

My friend had a situation that he want create a sheet that every forth month on the same date of billing the customer needs to be remained for their premium payment.  He want a function to use in his formula since their are 300 customer he need to send advice on next billing.

My Solution is;

Step 1 : Customer are need to be reminded for their premium payment on quarterly basis.  In a column put the billing date.

Step 2 : Create a column to find the date on which the reminder has to be send.

Step 3 : If  D1 has the billing date; the function/formula at E1 is;
=DATE(YEAR(D1),MONTH(D1)+3,DAY(D1))

Step 4:  Copy the formula for all the rows which will give you the next billing date.