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This is a common misconception among fans, and quite an easy one to make. Though Daniel did say shortly before his death that "this is our present...any one of us can die," some people may still not understand the logistics of Lost-ian time travel.
Daniel will still be born because he died in his present, even though it was before he was born. His life, though it did not occur throughout linear time, occurred in a linear way to him. He was born, he grew up, he came to the island, he was teleported back to the '70's, he died. His birth and death are two independent events, regardless of what time period they take place in.
I know that this blog post hasn't been particularly eloquent, but perhaps it clears up one of those murky misconceptions about Lostian time travel.