How time moves on the island vs off it

We know from season 4 that time does not behave the same way on the island that it does off it. Three instances prove this--when Faraday has the rocket fired from the ship, when the ship doctor's body washes ashore, and when the helicopter with Sayid and Desmond flies back to the ship.

Is it some kind of Interstellar-like wormhole which exists between the island and just offshore? Is one minute on the island equal to one hour off it, or vice versa? How does that work exactly?