Hello Ricardo,
I think you skipped the part, where I suggested that you contact your colleagues, who are SAP consultants in your company to help you build the logic.
You should have received some functional specification or at least some solution proposal explaining the process and from where to get the data, right? So you know the author and you can ask him for assistance if you don't even access ERP.
If you have no functional specification, then make sure to get one. You can't experiment by joining tables/fields and just ask random people in an internet forum until you get some meaningful results in your report.
It is also possible that your client has some developments and stores data in z-fields or z-tables, which SCN members will not know about, because they are specific to your company. They might be important for building your report and provide more effective (faster) way to get the information.
If you are absolutely sure that you are always getting the correct sales order number, then SD order creation date will be in VBAK-ERDAT (VBAK-VBELN is the sales order number).