How to change the base of the date in opencart?

Hi guys,

As I didn't receive any reply (even any views) on my last post, I tried a new subject!

Can you tell me how I can make the Opencart to show and probably work based on Persian Shamsi date? I have a php code which do the conversion of the date, I also added the date into invoice template (for receipt), but I want the cart to work based on this date, I mean I need the date to be shown on order history, invoices in client interface as well as admin front.

Thank you in advance,

Hope to hear from you soon