When are they created?
Radak: At the time they leave Hashem's mouth.
Malbim: In all previous miracles, Hashem did not create Yesh me'Ayin (something from nothing), rather, Yesh me'Yesh. The miracle was switching one form for another - a staff became a snake, water became blood, liquids stood, the sun ceased moving... The miracle was for a limited time; nature did not change permanently. The Navi needed to say when they will be. They were not Beriyah, rather, Yetzirah and Asiyah. When Hashem is Borei, even the magicians and witches will not attribute it to natural forces, stars or Mazalos. Even they think only that they form and make, but are not Borei.
Why does it say "v'Lifnei Yom v'Lo Shematam"?
Rashi: I inform you about them before they will be. V'Lifnei Yom refers to "Hishmaticha" (6). V'Lo Shematam - you did not hear them until today.
Radak: They were not created before today that I tell. Do not say 'we knew' - they were created today. You did not know or hear until now!
Malbim: The coming miracles will be Beriyah, so I will hide them from you. I will not tell the Navi beforehand, "lest you say 'we knew'", and they will not be a great Chidush to you.
