[ADD] purchase: How are scheduled order dates computed?

https://docs.google.com/document/d/1Ec6pmwA6MrEvi_hKpuBr8HbGflSeGsk8waWmN1id6KA/edit#heading=h.teahchhagxca

purchase/replenishment/flows/compute_date.rst

@@ -1,3 +1,156 @@
 ====================================================
 How are the order date and scheduled dates computed?
-====================================================
+====================================================
+
+Scheduled dates are computed in order to be able to plan deliveries,
+receptions and so on. Depending on the habits of your company, Odoo
+automatically generates scheduled dates via the scheduler. The Odoo
+scheduler computes everything per line, whether it's a manufacturing
+order, a delivery order, a sale order, etc. The dates that are computed
+are dependent on the different leads times configured in Odoo.
+
+Configuring lead times
+======================
+
+Configuring **lead times** is an essential move in order to compute
+scheduled dates. Lead times are the delays (in term of delivery,
+manufacturing, ...) promised to your different partners and/or clients.
+Configuration of the different lead times are made as follows:
+
+On a product level
+------------------
+
+Supplier lead time:
+~~~~~~~~~~~~~~~~~~~
+
+The supplier lead time is the time needed for the supplier to deliver
+your purchased product. To configure the Supplier lead time select a
+product (from the Purchase module, go to :menuselection:`Purchase --> Product`),
+and go in the **Inventory** tab. You will have to 
+add a **Vendor** to your product in order to select a supplier lead time.
+
+.. image:: media/compute_date01.png
+    :align: center
+
+.. tip:: 
+    It is possible to add more than one vendor per product and thus 
+    different delivery lead times depending on the vendor.
+
+Once a vendor is selected, click on it to open its form and indicate its
+delivery lead time. 
+
+.. image:: media/compute_date02.png
+    :align: center
+
+.. note:: 
+    In this case security days have no influence, the scheduled 
+    delivery days will be equal to: Date of the purchase order + Delivery Lead Time.
+
+Customer lead time
+~~~~~~~~~~~~~~~~~~
+
+The customer lead time is the time needed to get your product from your
+store/warehouse to your customer. It can be configured for any
+product. Simply select a product (from the **Sales** module, go to 
+:menuselection:`Sales --> Product`), 
+and go into the **Sales** tab to indicate your customer lead time.
+
+.. image:: media/compute_date03.png
+    :align: center
+
+On the company level
+--------------------
+
+On company level, it is possible to parameter **security days** in order
+to cope with eventual delays and to be sure to meet your engagements.
+The idea is to subtract **backup** days from the computed scheduled date
+in case of delays.
+
+Sales Safety days
+~~~~~~~~~~~~~~~~~
+
+Sales Safety days are **back-up** days to ensure you will be able to
+deliver your clients engagements on time. They are margins of errors for
+delivery lead times. Security days are the same logic as the early
+wristwatch, in order to arrive on time. The idea is to subtract the
+numbers of security days from the calculation and thus to compute a
+scheduled date earlier than the one you promised to your client. That
+way you are sure to be able to keep your commitment.
+
+To set up your security dates, go to the app 
+:menuselection:`Settings --> General settings`, 
+and click on **Configure your company data**.
+
+.. image:: media/compute_date04.png
+    :align: center
+
+Go the **Configuration** tab to indicate the number of safety days
+
+.. image:: media/compute_date05.png
+    :align: center
+
+.. tip::
+    Note that you can in this menu configure 
+    a default **Manufacturing** lead time.
+
+Purchase days
+~~~~~~~~~~~~~
+
+Purchase days response to the same logic than sales security days.
+
+They are margins of error for vendor lead times. When the system
+generates purchase orders for procuring products, they will be scheduled
+in order to cope with unexpected vendor delays. Purchase lead time can
+be found in the same menu as the sales safety days (see screenshot
+above).
+
+On route level
+--------------
+
+The internal transfers due to the movement of stocks can also influence
+the computed date.
+
+The delays due to internal transfers can be specified in the **Inventory**
+module when creating a new push rule for a new route.
+
+.. note:: 
+    Read the documentation 
+    :doc:`../../../../inventory/routes/concepts/push_rule`
+    to learn more.
+
+.. image:: media/compute_date06.png
+    :align: center
+
+On document level:
+------------------
+
+Requested date
+~~~~~~~~~~~~~~
+
+Odoo offers the possibility to indicate a requested date by the client
+straight on the sale order, under the tab **Other information**. If
+this date is earlier than the theoretically computed date, Odoo will
+automatically display a warning.
+
+.. image:: media/compute_date07.png
+    :align: center
+
+Example
+=======
+
+As an example, you may sell a car today (January 1st), that is purchased
+on order, and you promise to deliver your customer within 20 days
+(January 20). In such a scenario, the scheduler may trigger the
+following events, based on your configuration:
+
+-   January 19: actual scheduled delivery (1 day of Sales Safety days)
+
+-   January 18: receive the product from your supplier (1 day of Purchase
+    days)
+
+-   January 10: deadline to order at your supplier (9 days of supplier
+    delivery lead time)
+
+-   January 8: trigger a purchase request to your purchase team, since
+    the team needs on average 2 days to find the right supplier and
+    order.