Change colour once a certain time period passed

I record the date of when I send a letter. I send several hundred of these letters. Once 30 days passes, I need to send out a reminder to these people that receive the letters. I was thinking that perhaps the best way to do this would be to have the cells automatically change colour to indicate that 30 days has passed. But as only a basic formula user in excel, whats the simpliest way to do this?