این عین کدیه که خودم دارم استفاده میکنم
<asp:Label ID="Label1" runat="server" Text='<%# Deliver(Eval("trusteeship_date")) %>'></asp:Label>
و تابع:
public string Deliver(object trustship)
{
DateTime date = new DateTime();
date = Convert.ToDateTime(trustship);
Options o = new Options();
DataTable dt = new DataTable();
o.Id = 1;
dt=o.SelectOne();
date = date.AddDays(Convert.ToDouble(dt.Rows[0][3]));
PersianCalendar jc = new PersianCalendar();
string roz = Convert.ToString(jc.GetDayOfMonth(date));
string mah = Convert.ToString(jc.GetMonth(date));
string sal = Convert.ToString(jc.GetYear(date));
return roz + "/" + mah + "/" + sal;
}