Changing cell color based on date

[Muray]

Hello all, I run a physical therapy office and we need to follow up pateints based on the last visit. Our fancy office software doesn't do anything simple so I am hoping Excel could help us with this. So basically we want to have one column with all the active patients, and the column next to it with the last patient visit dates. Everyday we are going update the last visit a patient came to the office. If I can have the color of the cell change based on how long a patient had not been to the office, say, green for 7 days, yellow for 14 days, and red for 21+ day, it would help our office out greatly. I believe I need to do something called conditional formatting but I can't figure it out to save my life. Any help would be greatly appreciated, thanks!

[royUK]

Use Conditional Formatting

http://excel-it.com/excel_conditional_formatting.htm