1 Kings 9:10 – “And it came to pass at the end of twenty years, when Solomon had built the two houses, the house of the LORD, and the king’s house,”

Here we calculate with 20 full years as it is mentioned it was the end of these years: 2980 + 20 = 3000 AH.

We also should mention that God warned Salomon in the prior verses that his sin would lead to destruction. God never does anything without saying, as to give people a chance to repent:

1 Kings 9:6,7 – “But if ye shall at all turn from following me, ye or your children, and will not keep my commandments and my statutes which I have set before you, but go and serve other gods, and worship them: Then will I cut off Israel out of the land which I have given them; and this house, which I have hallowed for my name, will I cast out of my sight; and Israel shall be a proverb and a byword among all people:”

Later when Israel was cut off from the land, we take this as reference in our counting.