There is a posuk in tehillim, kapitl tzaddik, “ki elef shonim beiynecha kyom…”
Based on this, I think it is kabbola but may be a medrash that learn that just as Hashem created the world in six days with the seventh being shabbos, so too the world. Every day corresponds to a thousand years. The last millennium is moshiach which is shabbos.
But we don’t take Shabbos in at shkiah exactly, we light beforehand. So moshiach must arrive before then, and as noted we are currently erev shabbos after chatzos if you do the Mah.
As far as the possibility of another world: our chachomim in the gemara tell us that after this last millennium of 7000, no one knows what will be.
So whoever got scared, no worries till the year 7000, and in general no need to fear, as “ain ra yored min hashomayim” abs “Teva Hatov lhaitiv” so no matter what it is, it will be in our best interest