为什么我不能在asp.net上使用ToShortDateString

我曾经这样d.PODate.ToString()显示（请参阅图片），我可以使用inspect元素看到它……我只想显示日期而不是时间，当我尝试使用d.PODate.ToString("d")它时，给了我这样的错误Method 'System.String ToString(System.String)' has no supported translation to SQL，同样也d.PODate.ToShortDateString()得到了这样的错误Method 'System.String ToShortDateString()' has no supported translation to SQL.

编辑

这是我的GET方法

[HttpGet]
public Models.SysDataTablePager Get()
{
    NameValueCollection nvc = HttpUtility.ParseQueryString(Request.RequestUri.Query);

    string sEcho = nvc["sEcho"].ToString();
    int iDisplayStart = Convert.ToInt32(nvc["iDisplayStart"]);
    string sSearch = nvc["sSearch"].ToString();
    int iSortCol = Convert.ToInt32(nvc["iSortCol_0"]);
    string sSortDir = nvc["sSortDir_0"].ToString();

    var BranchId = Convert.ToInt32(((wfmis.Global)HttpContext.Current.ApplicationInstance).CurrentBranchId);

    var Count = db.TrnPurchaseOrders.Where(d => d.MstUser.Id == secure.GetCurrentUser() && 
    d.MstBranch.Id == BranchId).Count();

    var PurchaseOrders = from d in db.TrnPurchaseOrders
                         where d.MstBranch.Id == BranchId &&
                               d.MstBranch.MstUser.Id == secure.GetCurrentUser()
                         select new Models.TrnPurchaseOrder
                         {
                             Id = d.Id,
                             PeriodId = d.PeriodId,
                             Period = d.MstPeriod.Period,
                             BranchId = d.BranchId,
                             Branch = d.MstBranch.Branch,
                             PONumber = d.PONumber,
                             POManualNumber = d.POManualNumber,
                             //PODate = d.PODate.ToShortDateString(),
                             PODate = d.PODate.ToShortDateString(), //<------
                             SupplierId = d.SupplierId,
                             Supplier = d.MstArticle.Article,
                             TermId = d.TermId,
                             Term = d.MstTerm.Term,
                             RequestNumber = d.RequestNumber,
                             //DateNeeded = d.DateNeeded.ToShortDateString(),
                             DateNeeded = d.DateNeeded.ToString(), //<------
                             Particulars = d.Particulars,
                             RequestedById = d.RequestedById == null ? 0 :
                                             d.RequestedById.Value,
                             RequestedBy = d.MstUser.FullName,
                             IsClosed = d.IsClosed,
                             PreparedById = d.PreparedById,
                             PreparedBy = d.MstUser.FullName,
                             CheckedById = d.CheckedById,
                             CheckedBy = d.MstUser1.FullName,
                             ApprovedById = d.ApprovedById,
                             ApprovedBy = d.MstUser2.FullName,
                             IsLocked = d.IsLocked,
                             CreatedById = d.CreatedById,
                             CreatedBy = d.MstUser3.FullName,
                             //CreatedDateTime = d.CreatedDateTime.ToShortDateString(),
                             CreatedDateTime = d.CreatedDateTime.ToString(),
                             UpdatedById = d.UpdatedById,
                             UpdatedBy = d.MstUser4.FullName,
                             //UpdatedDateTime = d.UpdatedDateTime.ToShortDateString()
                             UpdatedDateTime = d.UpdatedDateTime.ToString()
                         };

    switch (iSortCol)
    {
        case 2:
            if (sSortDir == "asc") PurchaseOrders = PurchaseOrders.OrderBy(d => d.PODate).Skip(iDisplayStart).Take(10);
            else PurchaseOrders = PurchaseOrders.OrderByDescending(d => d.PONumber).Skip(iDisplayStart).Take(10);
            break;
        case 3:
            if (sSortDir == "asc") PurchaseOrders = PurchaseOrders.OrderBy(d => d.PONumber).Skip(iDisplayStart).Take(10);
            else PurchaseOrders = PurchaseOrders.OrderByDescending(d => d.PODate).Skip(iDisplayStart).Take(10);
            break;
        case 4:
            if (sSortDir == "asc") PurchaseOrders = PurchaseOrders.OrderBy(d => d.Supplier).Skip(iDisplayStart).Take(10);
            else PurchaseOrders = PurchaseOrders.OrderByDescending(d => d.Supplier).Skip(iDisplayStart).Take(10);
            break;
        default:
            PurchaseOrders = PurchaseOrders.Skip(iDisplayStart).Take(10);
            break;
    }

    var PurchaseOrderPaged = new Models.SysDataTablePager();

    PurchaseOrderPaged.sEcho = sEcho;
    PurchaseOrderPaged.iTotalRecords = Count;
    PurchaseOrderPaged.iTotalDisplayRecords = Count;
    PurchaseOrderPaged.TrnPurchaseOrderData = PurchaseOrders.ToList();

    return PurchaseOrderPaged;
}

编辑2

我在数据库的DateTime上得到了这个...