sales order Items having different billing date

HI experts,

we do have issue as follows:

• when we create a sales order with a billing block = Z1 or Z4 (collective invoice). Tha billing date for each item should have the same billing date (the maximun of the billing date of each item) , but it is not working like this.

• If sales order created with billing block Z1 or Z4, the billing dates for each line items should be same. But here in above case, the both line items created with different billing dates.

• The correction to be done when we have a Z1 or Z4 billing block is to check (and change if requiered) :

         * the billing date of each item when a new item is added on a sales order   

         * the billing date of each item if one billing date is changed   

         * check and change the billing date of each item when an item is deleted

• please suggest the posible solution.