BAPI_PO_CREATE1 can't create delivery date

I have specified the correct parameter but i still get Deliver_Date = sy-datum. So, I used BAPI_PO_CHANGE just to modify the delivery date. But why does po_create alone cannot specify the user-defined delivery date?

* Header
*  i_header-doc_date = sy-datum.            "Doc Date
  i_header-comp_code = c_bukrs.            "Company Code
  i_header-purch_org = 'AGPP'.             "Country
  i_header-pur_group = '001'.              "Purchasing Group
  i_header-vendorno = '100034'.            "Vendor No.
  i_header-currency = 'PHP'.               "Currency"
  APPEND i_header.
  CLEAR i_header.

* Item Details
  i_itemdtl-po_item = '00010'.
  i_itemdtl-material = ''.
  i_itemdtl-short_text = 'ticket Adult'.
  i_itemdtl-plant = 'AGP1'.
  i_itemdtl-quantity = '10'.  "*
  i_itemdtl-po_unit = c_ea.
  i_itemdtl-net_price = '10'. "*
  i_itemdtl-acctasscat = c_project.
* account
  i_itemdtl-gl_account = c_glact.
  i_itemdtl-wbs_element = c_wbs.
  i_itemdtl-delivery_date = '08/30/2007'.

  i_itemdtl-tax_code = ''.
  i_itemdtl-gr_rcpt = 'PF456'.
  i_itemdtl-unload_pt = 'MID111'.
  APPEND i_itemdtl.
  CLEAR i_itemdtl.


 DATA: lv_lifnr(10) TYPE n VALUE '0000000000',
        lv_saknr(10) TYPE n VALUE '0000000000'.



* transfer of data
  READ TABLE i_header INDEX 1.
* Header
  lt_header-comp_code = i_header-comp_code.       "Company Code
  lt_header-doc_type = 'NB'.                      "Doc Type
  lt_header-creat_date = '20070827'. "sy-datum.
"Created Date
  lt_header-created_by = sy-uname.                "Created By
  lt_header-item_intvl = '00010'.                 "Ite Interval

  lv_lifnr = i_header-vendorno.

  SELECT SINGLE lifnr
  INTO lt_header-vendor
  FROM lfa1
  WHERE  lifnr = lv_lifnr.



*    lt_header-vendor   = i_header-vendorno.         "Vendor No
  lt_header-langu    = sy-langu.                  "Language
  lt_header-purch_org = i_header-purch_org.       "Country
  lt_header-pur_group = i_header-pur_group.       "Purchasing Group
  lt_header-currency = i_header-currency.         "Currency"
*  lt_header-doc_date = i_header-doc_date.         "Doc Date
  APPEND lt_header.
*  CLEAR lt_header.


  LOOP AT i_itemdtl.
* item detail
    lt_detail-po_item =  i_itemdtl-po_item.
    lt_detail-material =  i_itemdtl-material.
    lt_detail-short_text =  i_itemdtl-short_text.
    lt_detail-acctasscat =  i_itemdtl-acctasscat.
    lt_detail-quantity =  i_itemdtl-quantity.
    lt_detail-po_unit =  i_itemdtl-po_unit.
    lt_detail-net_price =  i_itemdtl-net_price.
    lt_detail-matl_group = c_matkl.
    lt_detail-plant =  i_itemdtl-plant.
    lt_detail-tax_code =  i_itemdtl-tax_code.

* delivery
    lt_schedule-po_item = '00010'.
    lt_schedule-delivery_date =  i_itemdtl-delivery_date.
    lt_schedule-stat_date = i_itemdtl-delivery_date.
    lt_schedule-quantity = i_itemdtl-quantity.

* account
    lt_account-po_item = '00010'.
    lt_account-wbs_element = i_itemdtl-wbs_element.
    lv_saknr = i_itemdtl-gl_account.
    lt_account-gl_account = lv_saknr.
    lt_account-gr_rcpt = i_itemdtl-gr_rcpt.      "Proflight Code
    lt_account-unload_pt = i_itemdtl-unload_pt.  "Migrant ID


    APPEND: lt_detail,
            lt_schedule,
            lt_account.

    lt_headerx-comp_code = c_true.        "Company Code
    lt_headerx-doc_type = c_true.        "Doc Type
    lt_headerx-creat_date = c_true.       "Created Date
    lt_headerx-created_by = c_true.       "Created By
    lt_headerx-item_intvl = c_true.       "Ite Interval
    lt_headerx-vendor   = c_true.         "Vendor No
    lt_headerx-langu    = c_true.         "Language
    lt_headerx-purch_org = c_true.        "Country
    lt_headerx-pur_group = c_true.        "Purchasing Group
    lt_headerx-currency = c_true.         "Currency"
*    lt_headerx-doc_date = c_true.         "Doc Date
    APPEND lt_headerx.

    lt_detailx-po_item =   '00010'.
*    lt_detailx-material =   c_true.
    lt_detailx-short_text =  c_true.
    lt_detailx-acctasscat =   c_true.
    lt_detailx-quantity =   c_true.
    lt_detailx-po_unit =   c_true.
    lt_detailx-net_price =   c_true.
    lt_detailx-matl_group =  c_true.
    lt_detailx-plant =   c_true.
    lt_detailx-tax_code =   c_true.

    lt_schedulex-po_item = '00010'.
    lt_schedulex-sched_line = c_True.
    lt_schedulex-delivery_date =  c_true.
    lt_schedulex-stat_date = c_true.
    lt_schedulex-quantity = c_true.

    lt_accountx-po_item = '00010'.
    lt_accountx-gl_account = c_true.
    lt_accountx-wbs_element = c_true.
    lt_accountx-gr_rcpt = c_true.
    lt_accountx-unload_pt = c_true.

    APPEND: lt_detailx,
            lt_schedulex,
            lt_accountx.


*** Fillup the transaction
  CALL FUNCTION 'BAPI_PO_CREATE1'
    EXPORTING
      poheader         = lt_header
      poheaderx        = lt_headerx
    IMPORTING
      exppurchaseorder = po_number
    TABLES
      return           = return
      poitem           = lt_detail
      poitemx          = lt_detailx
*      poschedule       = lt_schedule
*      poschedulex      = lt_schedulex
      poaccount        = lt_account
      poaccountx       = lt_accountx.

  COMMIT WORK AND WAIT.

  IF po_number IS NOT INITIAL.
    CALL FUNCTION 'BAPI_PO_CHANGE'
      EXPORTING
        purchaseorder = po_number
      TABLES
        return        = return
        poschedule    = lt_schedule
        poschedulex   = lt_schedulex.
    COMMIT WORK AND WAIT.
  ENDIF.