These are jujubes.
We find them mostly in the Mediterranean region starting from like 500 m height(I have one of these tree in my garden).
You should just be careful if there is no holes made by worms in there, then you souldn't eat it. It's not poisonous but it will probably taste bad. To know, you bite it in the middle to check.
It grows end of august, start of september.
It's not too sweet unless it's very wrinkled.