Time is important on the island but not to the rest of the world. Whatever powers the island has slows down time only on the island. Thats why Daniel's rocket showed 31 minutes ahead of the island time. But time moves regualrly everywhere else. Remember Ben showed Jack that Boston won the world series. So if we assume that for every hour of island time equals two hours of world time. so 3 months on the island is 6 months world time. Thats enough time to have a baby even if its pre-mature. So Aaron can pass as Kate's son.

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