That happens because IIS is running with a different locale which uses a different DateTime format.
You must tell the parser which format to use. E.g.
System.Globalization.DateTimeFormatInfo dtf = new System.Globalization.DateTimeFormatInfo();
dtf.ShortDatePattern = "dd/MM/yyyy";
string input = "20/11/1975";
DateTime result = DateTime.Parse(input, dtf);