This is a "Kai Yuan Tong Bao" cash coin; these were issued throughout the Tang dynasty, for nearly 300 years (621 to 907 AD). Experts have identified certain varieties and attributed them to certain specific time periods, but I'm far from expert. "Tang dynasty" is all that can be said for certain.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis