On the 22d of August he knew that Howe had really gone southward and was in Chesapeake Bay.

Boston was now certainly safe.

On the 25th of August, after three stormy weeks at sea, Howe arrived at Elkton, at the head of Chesapeake Bay, and there landed his army.

It was Philadelphia fifty miles away that he intended to have.
Washington wrote gleefully "Now let all New England turn out and crush Burgoyne." Before the end of September he was writing that he was certain of complete disaster to Burgoyne.
Howe had, in truth, made a ruinous mistake.

Had the date been May instead of August he might still have saved Burgoyne.