Ok, Lost fans. Here's my theory for the time displacement surrounding the island:
Picture the island as a dot on a piece of paper. Picture the boat as another dot on the other end of the same side of the paper. Now draw a line between the two dots - that's the distance in real time and space between the island and the boat:
The chopper takes off from the island around dusk (according to Sayid). It's dusk on the boat, too, because the two areas are actually in real time sync with each other. It's Christmas-ish 2004 in both places. Things are proceeding normally.
Now picture the island existing within a bubble or dome of some sort. Anything that hits this bubble without being on the precise correct heading (sort of a corridor through the bubble) encounters a "warp". This "warp" connects through "subspace" (for want of a better term) and bridges the 40-minute gap. While in that warp, for the people on the chopper a short but indeterminate time has passed, but it's obvious when they finally get through and arrive at the boat, it's the middle of the next day. It's also the middle of the next day on the island. So they've essentially been caught up in that warp for a day, while subjectively to them the time passes much more quickly. Picture it as a layer of jello that, while allowing radio waves to pass through unhindered (they can talk in real-time from the island to the boat and back on the radio) it hinders matter and slows it down. The effect is more or less unnoticed on the chopper by Frank and Sayid (although they experience it as storm turbulence) except for Desmond, who, due to his unique exposure to the hatch electromagnetic radiation, becomes "unstuck" in time.
So the chopper and its passengers leave the island at dusk, run into a thunderhead, and come out of it on the other side in full daylight. To them, only a short time has passed but to the island and the boat it's been a full day.
This accounts for what happened to Daniel's rocket in another way - it was launched from the boat, and supposed to arrive on the island in app. 20 seconds. It takes far longer than that to arrive, and when their clocks are compared, it shows that while "x" minutes have passed for Daniel on the island, "x+31" minutes have passed for the rocket. It was stuck in the barrier for a number of minutes, out of time with the rest of the island.
So picture the barrier as a big bubble of jello, catching objects going in and out and holding them for a particular amount of time that seems short to those coming out and long to those coming in, then releases them to arrive at their destinations. If Frank flew the chopper back to the island and missed the safe corridor, he would probably arrive later than expected again, and to Frank he would've been gone even longer still.