It looks like that in the Original Hebrew Gen 8:4, and in KJV for example, the date given is the seventeenth of the seventh month
וַתָּנַח הַתֵּבָה בַּחֹדֶשׁ הַשְּׁבִיעִי, בְּשִׁבְעָה-עָשָׂר יוֹם לַחֹדֶשׁ, עַל, הָרֵי אֲרָרָט
And the ark rested in the seventh month, on the seventeenth day of the month, upon the mountains of Ararat.
While in the Vulgate (and I think also in the Septuagint) the date mentioned is the twenty-seventh.
requievitque arca mense septimo vicesima septima die mensis super montes Armeniae (Douay-Rheims English Translation: And the ark rested in the seventh month, the seven and twentieth day of the month, upon the mountains of Armenia.)
I wonder what is the source of the this discrepancy?