using range validater its easy to validate the Only accept futuredate filed

//in your page load add following codes

rvDelivery.MinimumValue = DateTime.Now.ToShortDateString();
rvDelivery.MaximumValue = DateTime.MaxValue.ToShortDateString();

//Add a customer validator to the page and change this values only
controleto validate : your txtboxname
validation group: your validation group
type: Date