In the external databound event just set up your switch statement and compare your stored date to the current date. Or compare it to the currentdate and add(-30 days) so and if yours is greater, then it passes. Kentico has examples of this everywhere, and you mimic their onexternaldatabound events in their unigrids.
Once your comparison is made, then you can change the color of the row by possibly using the gridviewrow style attribute and use a css class to set the background.
GridViewRow.Style.Add("CustomRedClass");