Why is the coming Rosh Hashana Year of 5778 six months late, that is the Rosh Hashana Year of 5778 actually began on 1 Nisan, as according to the following Torah Passage:
Shemot 12:2 הַחֹ֧דֶשׁ הַזֶּ֛ה לָכֶ֖ם רֹ֣אשׁ חֳדָשִׁ֑ים רִאשׁ֥וֹן הוּא֙
לָכֶ֔ם לְחָדְשֵׁ֖י הַשָּׁנָֽה
The Torah calls the month of Nisan "Rishon HaShanah" (Shemot 12,2) רִאשׁוֹן... הַשָּׁנָה.
The answer I believe (the italics below is mine) is:
"from the Sefer Or haChammah. A midrash in Bereishit Rabbah 10:4
states that the planets and spheres traveled at an extremely rapid
speed prior to the time of Adam’s sin (on the sixth day of creation).
According to this explanation, the planets traveled so fast that they
completed a six month journey in an actual time frame of two days
between the placement of the heavenly bodies in the firmament on the
fourth day of creation in tekufat Nissan, and the time of the creation
of man on the sixth day of creation. Consequently, even thought the
world was created on the first day of Nissan, the sixth day of
creation actually took place on the first day of Tishrei."
The Rabbis, our Sages, created our calendar dating system, for example, 5778, based on and from the creation of the world, which they believe began on Nisan. The introduction of this rabbinical calendar dating, which tomorrow will be 5778, in the month of Tishrei, which the rabbis have every prerogative to do, is nevertheless six months late.