SAP SD Princing How pricing date is determine in the sales order and billing document? Where is the setting?

The pricing date is proposed based on the setting you make in the Sales document configuration. ( T code : VOV8)
You have a field" Prop.f.pricing date " in the Requested delivery date / pricing date / purchase order date segment.

Then you can choose the follwoing options:
Blank - Indicates the current date as the pricing date
A - Indicates the date based on the requested delivery date
B - Indicates the date based on the order validity start from date

And the pricing in the billing document is copied from thte sales order / Delivery document..

It again depends on the setting u have in the copy control from order - billng or delivery - billing.
In the copy control, in the item settings you have two fields relavant for this.

One is pricing source and the other is pricing type.

The pricing sources are generally the order. But if you want you can change it to other values mentioned in the drop down,
but this values have no effect if the pricing type is B.

Any other value other than B in the pricing type will take the reference document price mentioned in the pricing source field.
but for the pricing type B. The new price is determined in the billing order.