When an order/offer is created, it will store the date and time in the table sale_order, field create_date.

When an order/offer is modified, it will store the date and time of the last modification in the same table, field write_date.

So you probably want to display at least one of these fields, depending if you want to see the latest modification or the original date.

Steps to take:

• create new module
• create an XML-file, which extend sale.order.form, and add the field(s) you want to display
• upload the module to the /addons directory (location depends on the OS)
• install your module

For all these steps you can find (some sort of) system documentation (official openERP documentation, sometimes a bit cryptic), all kind of questions here, or post a question....