Hi...
Regarding the exception...
Check if you actual have field with name "DATE" in the reader. Try to assign value to a simple textbox. Still if u get exception then it's clear the reader is not having a fields with name "DATE".
Regarding ToString... try following..
txtDate.Text = String.Format("{0:dd/MM/yyyy}",reader["DATE"]);
This should work for you.
Thanks and regards,
Chetan Ranpariya