Actually.... the 31 day clause comes into effect, after Congress is notified of intentions, and in the event of national defense.
Obama made the assertion in a Dec. 20, 2007
interview with the Boston Globe when reporter Charlie Savage asked him under what circumstances the president would have the constitutional authority to bomb Iran without first seeking authorization from Congress.
“The President does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation,” Obama responded.
“As Commander-in-Chief, the President does have a duty to protect and defend the United States,” Obama continued. “In instances of self-defense, the President would be within his constitutional authority to act before advising Congress or seeking its consent. History has shown us time and again, however, that military action is most successful when it is authorized and supported by the Legislative branch.”