You seem to be missing nolu chan's point. Equally relevent: What is the birthdate of AT&T? Is their corporate structure unchanged from the old Bell Labs days?
Implicit in most pro-secessionist arguments is that, after the Treaty of Paris in 1783, the United States were 13 independent, sovereign, entities working together in a loose coalition, league, or confederation. Sovereignty, they say, rests solely with the States. Some even go the the lengths of purposely refering to the "united States", emphasizing "united" as an adjective, rather than "United States" as a collective noun; denying the existance of the coountry.
My point is that the United States has been, since July 1776, an independent nation. American nationalism pre-dates the Declaration of Independence, expressed not only in documents such as the "Articles of Association," but in attitudes and collective experiences as British colonies, forced because of geography, to be increasingly self-sufficient. Americans had abundant experience in intra-colonial, as well as inter-colonial government prior to independence. Indeed, independence was driven, in part, from their past governing experiences.
I see the development of the American nation to be expressed in the development of a "national" (Madison = "general") government. From 1776 to 1781 the States did act autonomously, but cooperatively. But as early as 1777, the groundwork for the Articles of Confederation had been laid. Under the British Crown, sovereignty rested with the King. Americans decided, and expressed in their Declaration, that sovereignty rested with the people. When the people form representative governments, they not only empower those governments to do certain things, they transfer a degree of sovereignty to the government. (The concept of sovereignty implies not only supremecy and authority, but also the ablity to govern.)
The Articles of Confederation, when ratified in 1781, created a weak central authority, which was, for all purposes, a "national" government. (The term, "The United States of America" first shows up in the Articles.) Certain powers, that heretofore had been reserved by the individual States, were transfered to the confederal government. If the State can be considered to be "sovereign" and "independent" and capapble of self-rule, then too can a "national" government. Under the Articles, the States did not cede much, due in part to Americans understandable distrust of strong central authority.
By 1787 it was clear the Articles contained fatal defects. The Framers of the Constitution, who had ample experience in writing State constitutions, determined to replace, rather than repair the Articles. It is interesting to note, however, on the basis of textural analysis, nearly 2/3rds of the provisons in the Articles are preserved in the Constitution and the Bill of Rights.
In changing the form of government from a confederation with a weak central authority to a federal system with a stronger, but limited, authority, the Framers sought to reach a compromise between the well-established "sovereignty" of the States and the unexpressed sovereignty of the people. Under the Constitution, the States would retain sovereignty in State affairs. The federal government, authorized by the people, would exercise sovereignty in "national" affairs. The people would govern through representatives (a Republican form of government), including a national executive authority as well as State and federal legislators. Each level of authority, from the people through the federal government, possessing the prerogative to rule based on establish principles in written documents (i.e the rule of law). The Constitution is an expression of American nationalism, rather than a mere compact between State governments. Once established, no State possessed the right to unilaterally dissolve or exit the Union their people had ratified. The people retained the natural right of revolution if they suffered oppression; this is unchallenged. But in over 140 years I have yet to see a convincing case made that the secessionist states were truly oppressed. What I have seen is the attempt to conflate the right of revolution with the so-called right of secession.
Changing a form of government does not automatically create a new country. In our own history we see the progression from a cooperative, to a confederatioon, to a constitutional union. Nationality is more than government. This was recognized by Madison, Jefferson, Hamilton, et al. It is relevent what the people record as the birthdate of their country, because it provides a benchmark in time in the development of a nation. The American nation, declared in July 1776, exists because it is the will of the people that it do so.