Try using the following lines and let me know if there is any other issue:
DateTime elshose;
bool elshoseOkay;
elshoseOkay = DateTime.TryParse(Data["tahunterbit"].ToString(), out elshose);
if (elshoseOkay)
{
txttahun.CustomFormat = "yyyy";
txttahun.Format = DateTimePickerFormat.Custom;
txttahun.Text = elshose.Year.ToString();
}