Here's a source for functions that you will likely find useful http://www.techonthenet.com/access/functions/
Usually there are many ways to calculate something. Here's one related to your question: DateAdd("m",-4,Date)
This will provide a date 4 months prior to today. To alter the period, the m (month parameter) can be changed.
To alter the date, you'd supply an actual formatted date (e.g. DateAdd("m",-2,#06/23/2016#)
or a reference to a form or query field that provides a date (e.g. DateAdd("m",-2,[SalesDate])
In the example, Date is a reserved word for the Date function, so do not use "Date" anywhere in your db to name any object or field.
As for the email part, I presume you know how to handle that because you provided no information around those requirements.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.