c#

Just try

dd/mm/yyyy Format In regular Expression (optional mm,optional dd)
([1-9]|0[1-9]|[12][0-9]|3[01])[- /.]([1-9]|0[1-9]|1[012])[- /.][0-9]{4}$


mm/dd/yyyy (optional mm,optional dd)
^([1-9]|0[1-9]|1[012])[- /.]([1-9]|0[1-9]|[12][0-9]|3[01])[- /.][0-9]{4}$

mm/dd/yyyy (Exact Format)
^([01]\d)/([0-3]\d)/(\d{4})$

OR Try this function
  private void DateSubmit_Click(object sender, System.EventArgs e)
  {
   DateTime date1, date2;
   bool date1OK, date2OK;
   date1 = new DateTime(1,1,1);
   date2 = new DateTime(1,1,1);
   try
   {
    date1 = Convert.ToDateTime(Date1.Text);
    date1OK=true;
   }
   catch
   {
    date1OK = false;
   }
   try
   {
    date2 = Convert.ToDateTime(Date2.Text);
    date2OK=true;
   }
   catch
   {
    date2OK = false;
   }
   if(date1OK && date2OK)
   {
    if(date1.CompareTo(date2) > -1)lblResult.Text = "Date2 must be after Date1";
    else
    {
     //do whatever here
    }
   }
   else lblResult.Text = "Invalid date format. Please check your input in the Date1 and Date2 fields";
  }