Hello S(?),
If you have the date you want to post in a database table, you don't need this badi. Also posting date development is very much related to ERP-LE.
Create an enhancement in FM: BAPI_OUTB_DELIVERY_CONFIRM_DEC. Get posting date from EWM using an RFC FM and update deadlines.
Implement BAdI: LE_SHP_GOODSMVT. Influence your posting date here.
Note: This method can also be used for GR posting date.
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Best regards
Serhan