Skús niečo na tento spôsob (netestované):

ConnectionStringSettings strConStr = ConfigurationManager.ConnectionStrings["TISKARNYConnectionString"];
List<DateTime> dateTimes = new List<DateTime>();

using (IDbConnection myConnection = new SqlConnection(strConStr.ConnectionString))
{
	myConnection.Open();
	IDbCommand myCommand = myConnection.CreateCommand();
	myCommand.CommandText = "SELECT Datum FROM tiskarny";

	using (IDataReader reader = myCommand.ExecuteReader())
		while (reader.Read())
			dateTimes.Add(reader.GetDateTime(0));
}

DateTime[] kontDatum2 = dateTimes.ToArray();