The solemn ratifications of the present treaty expedited in good and due form shall be exchanged between the contracting parties in the space of six months or sooner, if possible, to be computed from the day of the signatures of the present treaty. In witness whereof we the undersigned, their ministers plenipotentiary, have in their name and in virtue of our full powers, signed with our hands the present definitive treaty and caused the seals of our arms to be affixed thereto. (10.1-2)

This chunk o' text can be found in Article 10, which stipulates that the treaty has to be ratified. This is because treaties can't really exist in a vacuum. The representatives have the authority to negotiate them, but to actually make them law? That's the responsibility of lawmakers.

The treaty gives a time limit because it's always a good idea to get war officially over with as soon as possible. A "kinda-sorta-maybe-it's-still-happening" war is good for exactly 0% of the population.