"Where in the Constitution of the United States does it say that the Congress decides how long people will spend on tours of duty and how long they will spend back in the United States? It's blatantly unconstitutional," McCain said. "The Constitution of the United States said Congress will declare wars and fund wars."
Will and McCain need to re-read Article 1, Section 8. The following power is explicitly granted to Congress:
To make Rules for the Government and Regulation of the land and naval Forces
There is no ambiguity here.