//和暦変換
CultureInfo c = new CultureInfo("ja-JP");
DateTimeFormatInfo dtf = c.DateTimeFormat;
dtf.Calendar = new JapaneseCalendar();
try{
DateTime d = DateTime.Parse(textBox3.Text);
textBox4.Text = d.ToString("ggyy年M月d日(ddd)", c);
}
catch (Exception ex) {
textBox4.Text = ex.Message;
}