Lets get more technical then.
Federal law and my history classes in elementary school states that "...You must be a natural-born citizen to become President.."
Federal law states the following for the Panama Canal Zone: "Any person born in the Canal Zone on or after February 26, 1904, and whether before or after the effective date of this chapter, whose father or mother or both at the time of the birth of such person was or is a citizen of the United States, is declared to be a citizen of the United States.
This did not become the law until August 4, 1937 after he was born.
So he was not a US born citizen for the first year of his life. At least that is how I interpret it as.