
Relative References
You can reference cells by how far away they are from the current cell, not just by their date. You can reference a cell one day older than the current cell, one week older, two weeks older, and one month older. While you are editing a formula, you can bring up a contextual menu by right-clicking, or holding down the Control key and clicking.
Then select the reference you want to use by clicking on that cell in the calendar.