It's counterfeit, but much better than the typical counterfeit halfpenny because a 1780s date is not common. It's worth looking into because it's worth in the range of anywhere from $100 - $500, or who knows. Chances are it's going to be the low end, but new varieties still get discovered and if it was say something like a mule not seen before it would easily be worth more than $500. I remember having a similar one, maybe the same coin, and the collectors also collected by die state, and I was lucky I had the least common (die state) and sold it for a fair chunk of change.
PS: I may be quite conservative with the numbers I am using. It's a good coin, it's just that it's like any other coin, little things matter and it needs to be Id'd.
I think the date is likely 1785.