Is there a RangeAttribute for DateTime?

Use this attribute:

public class CustomDateAttribute : RangeAttribute
{
  public CustomDateAttribute()
    : base(typeof(DateTime), 
            DateTime.Now.AddYears(-6).ToShortDateString(),
            DateTime.Now.ToShortDateString()) 
  { } 
}

Even though there is an overload for Range attribute that accepts type and boundary values of that type and allows something like this:

[Range(typeof(DateTime), "1/1/2011", "1/1/2012", ErrorMessage="Date is out of Range")]

what you are trying to achieve is not possible using this attribute. The problem is that attributes accept only constants as parameters. Obviously neither DateTime.Now nor DateTime.Now.AddYears(-6) are constants.

However you can still do this creating your own validation attribute:

public class DateTimeRangeAttribute : ValidationAttribute
{
    //implementation
}