The real date the U.S. Constitution came into existance

The document was completed in 1787. But it did not take effect until nine states had ratified it. It was adopted by the constitutional convention on September 17, 1787. The Continental Congress sent it to the states for ratification on September 28, 1787. The ninth state to ratify it was New Hampshire, in June, 1788. So, technically, it went into effect in 1788. However, it replaced the Articles of Confederation which required the unanimous approval of the states for certain measures, so many Americans did not feel it took effect until all thirteen states had ratified it. Rhode Island did not ratify until May, 1790. I don't know where anyone comes up with the 1789 date, unless it is that a government was formed based on the ratification of nine states, and the First Congress sat for the first time on March 4, 1789.

The Philadelphia Convention was in reality a constitutional convention. The convention actually met on May 15, 1787 in the Independent hall and unanimously selected George Washington as the chairman of the convention.

That is totally irrelevant to the question asked. The building to which you incorrectly referred is Independence Hall. So what are you, an inept content bot?