Need to validate a date entry for a SharePoint 2010 list item for 30 day window?

I need to ensure that a user has to enter a date that is within 30 days from [Today]. So in other words:   Date <= [Today]-30

What is the best way to do this? I need to make sure they are entering items in that are NOT older than 30 days from today's date? (I also would then need to make sure they cannot edit items that are more than 30 days old in the same list? What is the best way for this? I would like the KISS method if possible. ) :)

Any ideas would be fabulous!

KZ