You need to go to:
SPRO->IMG->CRM->Transactions->Basic Settings->Define Transaction Type.
Then find out your target transaction type and note the date profile.
Then Go to:
SPRO->IMG->CRM->Basic Functions->Date Management->Define Date Profile
Navigate to your date profile and look at the date types maintained.
Note the one relevant for Valid To date.
Also similarly for your source document find out the date type.
In the BADI, we need to copy the TIMESTAMP_FROM and TIMESTAMP_TO dates from source document date type to target document date type. Check if this information is available in the BADI in any of the strcutures. If they are not available we need to do an order read using CRM_ORDER_READ function. You may use requested object as APPOINTMENT.
Once the source date type values are found, we would populate the CS_DATE structure. Just make sure we get the correct target date type.
Hope this helps.