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Albus' words are not definitive on whether the boy was born already or was only due to be born. For him to be referring unambiguously to a yet-to-be-born child he should have said 'the prophecy is about a boy who will be born at the end of July' or 'the prophecy is about a boy due at the end of July'. What in Severus' words indicates to you the boy wasn't born yet?
Late 1980 - early 1981 matches the timeline of arrests/deaths of DEs: Moody started following Igor in early 1981, 6 months before Igor's arrest, which took place after the deaths of the McKinnons (in July, not long before Harry's birthday) but before the deaths of the Prewett brothers and the others in the Order photo, also before the death of Evan Rosier and the arrests of Dolohov, Travers and Mulciber. If those arrests and one death of DEs are the result of tip-offs by Severus (implied by Severus being known to Igor) then this places the beginning of Severus' spying in early 1981 (more than half a year before he started teaching).
In any case, I don't believe even Voldemort would choose whom to attack before he had a definite birth date. Had Lily gone into labor prematurely or a day later Harry wouldn't have matched the terms of the prophecy.
Late 1980 - early 1981 matches the timeline of arrests/deaths of DEs: Moody started following Igor in early 1981, 6 months before Igor's arrest, which took place after the deaths of the McKinnons (in July, not long before Harry's birthday) but before the deaths of the Prewett brothers and the others in the Order photo, also before the death of Evan Rosier and the arrests of Dolohov, Travers and Mulciber. If those arrests and one death of DEs are the result of tip-offs by Severus (implied by Severus being known to Igor) then this places the beginning of Severus' spying in early 1981 (more than half a year before he started teaching).
In any case, I don't believe even Voldemort would choose whom to attack before he had a definite birth date. Had Lily gone into labor prematurely or a day later Harry wouldn't have matched the terms of the prophecy.