As you all know Compare validators can be used to validate dates and check based on operator type (<, <= , >= etc). I have set the cultureinvariantvalues=\"true\
Try custom Validator and at the code behind at onservervalidate event convert the text to DateTime and then do the comparision.
protected void DateTimeComparision_ServerValidate(object source, ServerValidateEventArgs args)
{
args.IsValid = Convert.ToDateTime(txtStartDate.Text) < Convert.ToDateTime(txtFinishDate.Text);
}