Dear Friend,
Rather applying
DateTime.ParseExact("YYYY-MM-DD",dr[3])
you should apply it as
DateTime.ParseExact(dr[3].ToString(),"YYYY-MM-DD",CultureInfo.InvariantCulture)
Please follow this link for more details:-
http://www.dotnetperls.com/datetime-parseexact[
^]
I hope this will help you out.
Thanks