From the day that they built it. It is useless to tell an impassioned orator that his words are not strictly consistent with primitive history. The Israelites may not have built Jerusalem, but Jeremiah was not to be debarred from the strongest form of expression open to him for such a reason. He means "from the earliest times."
Repeated, with slight variations, from Jeremiah 7:30, Jeremiah 7:31. "Baal" and "Molech" are identified as in Jeremiah 19:5 (= Jeremiah 7:31), and even more distinctly.