The approximate date varies, with 100AD for Jehovah Witnesses and 312AD for Calvinists and Mormons.
The Church of Jesus Christ of Latter Day Saints (Mormons is a nick name and we would rather it not be used) belives that the apostacy was a gradual process and do not attach any specific date to when the original church went apostate. You might want to read 'The Great Apostacy' (check a local LDS bookstore) for a detailed arguemnt that an apostacy took place.
Two: If the Spirit indeed does confirm that the NT cannon is correct, then one has to admit that the either an apostate Church produced an infallible NT cannon (a contradiction) OR, that in fact, the Church wasn't apostate after all.
Those are not the only possible conclusions. You could also consider the possibility that the apostate church composed an tainted NT that is mostly as it was originaly, but not completely like what the apostles wrote, so it still contains much of God's word, but is not perfect (leading to much contention and disagrement among Christians for 2000 years).
True, but I suspect all of us here would not accept that conclusion. But it is an alternative.