It is important to remember that the Torah does not tell us when the world was created. And if you think that it is nevertheless clear that the Jewish tradition holds that Rosh Hashanah is the date of the world’s creation, you are mistaken. Not only is this a point of contention in the Talmud (Rosh Hashanah 10b), there are good reasons to say that the dominant position is that of Rabbi Yehoshua, who posits that the world was created on the first day of Nissan. On the other hand, you would not be mistaken to say that the Rosh Hashanah liturgy seems to indicate that we follow Rabbi Eliezer, that it was created on the first day of Tishrei.
A popular resolution of this problem is first found in Tosafot (Rosh Hashanah 27a) in the name of Rabbenu Tam. The world was conceived on the first day of Tishrei, but only physically created on the first day of Nissan. Such an idea is nicely rooted in the physical world, at least in the Northern hemisphere. To see more, go to the Jewish Press