Relative dates based custom groups

The ability to create custom groups based on "today" instead of a static date.

I have a group of people with custom date information. I would like to create a smart-group that takes today's date, adds or subtracts a certain amount of years, months, and days, and then gives me a list of people who's custom data corresponds to the calculated date. As today's date changes, so would the membership of the smart group.

The current way that I handle this is to create the smart group based on a static date, then update it periodically by hand.