ATP integration between APO and R3 is a mess. Whenever possible, following this simple recipe avoids many problems:
1) Don't set any restriction on receiving hours at the customer
2) Change the default requested time in sales orders to some normal value like 9 am.
In sales order the proposed delivery time can be changed on exit MV45AFZZ, in userexit_move_field_to_vbep. For stock transport orders it can be changed in BADI ME_PROCESS_PO_CUST.