Mac backup options

Can someone please help me.

Currently I take mirror copies of important folders/files and store offsite. Each time I need to take a complete mirror copy of the data which is very time consuming.

Is there a backup utility that will make a complete copy of selected folders/files, and then only back-up the folders/files where change has taken place. I would like to have one copy offsite (A), and another copy onsite (B) which I regularly update. Then say every month, swap the onsite copy (B) with the offsite copy (A). I then want to bring (B) up to date and continue the process i.e. regularly update changes to (B), then after a month swap (B) with (A) and repeat.

How do I do this? Is there an app or utility that can do this?