I'm going agree that Jin were flashbacks and Sun's were flashforward, but is the date that's on Jin's headstone the fake plane crash day or is it the day of something yet to come?

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Standing in the ocean with the sun burning low in the west
Like a fire in the cavernous darkness at the heart of the beast
With my beliefs and possessions, stopped at the frontier in my chest
At the edge of my country, my back to the sea, looking east