How do I get the date for the first and last day of the week, and the first and last day of last week?

I have a date range selection drop down list. I want the user to be able to pick options such as Today, Yesterday, This Month, Last Month, and have two date fields below the drop down populate with the appropriate dates. I have two columns in the table used to display the date range name, DateStart, DateEnd. In each I have some code such as =Date() for Today. I can get some simple VBA code to pull =Date() out and put it into the text box, but...