End date can't be prior to start date

@reluctantprogrammer 

You can write a simple validation on the end date field exit as below:

 form1.#subform[0].DateField2::exit - (JavaScript, client)
  var startDate = new Date(DateField1.rawValue);
  var endDate = new Date(DateField2.rawValue);   
      if(startDate  > endDate )
{
 xfa.host.messageBox("End Date:"+ endDate +"should be greater than Start Date:"+ startDate);
}

You can add more validations to this check to ensure startDate is not null, and any change in the date fields will retrigger validations etc.

Hope this helps!