Hello Kumar,
I hope this help:
To add the start and end date to the contract using this BAPI you need add item "POSNR" 000000 to filed the date on the header of the contract.
Try this:
wa_contract_data_in-itm_number = '000000'.
PERFORM f_convert_date_to_internal USING ls_header_data-value_0009
CHANGING wa_contract_data_in-con_st_dat.
PERFORM f_convert_date_to_internal USING ls_header_data-value_0010
CHANGING wa_contract_data_in-con_en_dat.
IF wa_contract_data_in IS NOT INITIAL.
APPEND wa_contract_data_in TO bapi-contract_data_in.
FREE wa_contract_data_in.
wa_contract_data_inx-con_st_dat = abap_true.
wa_contract_data_inx-con_en_dat = abap_true.
APPEND wa_contract_data_inx TO bapi-contract_data_inx.
FREE wa_contract_data_inx.