Why is Rosh Hashanah decribed as "covered up" or obscure? תִּקְעוּ בַחֹדֶשׁ שׁוֹפָר בַּכֵּסֶה לְיוֹם חַגֵּנוּ?

What if you hold the world was created in Nissan? And anyway, the story of Moshe Rabbenu pleading before Hashem a third time (Elul-Tishrei) is very unclear in the text of the Chumash.