The Sages disagree about when the world – or more precisely when the first human – was created. R. Yehoshua maintains that it was the first of Nisan, as the Torah refers to Nisan as the first month. R. Eliezer maintains that it was the first of Tishrei. This disagreement reflects the hidden character of Rosh Ha-shana, which leads to a dispute about what happened on its date. The Rishonim explain that both opinions are correct: God thought about creating the world on the first of Tishrei, and actually created it on the first of Nisan. The disagreement is about which day we should consider primary: the day that God, as it were, thought of creating the world, or the day He actually created it (Rabbeinu Tam). The Sages tell us that we follow R. Eliezer in practice, which is why the Rosh Ha-shana prayers read: “This day is the beginning of Your works, a commemoration of the first day” (Rosh Ha-shana 27a and Tosafot ad loc.). In any event, all agree that God judges His world and creates the new year on the first of Tishrei. This is why it is called “Rosh Ha-shana” (literally, the head of the year), as everything which happens in the course of the year is a result of what happens then.
The halakhic significance of Rosh Ha-shana as the “new year” pertains to the dating of contracts, counting years for Shemita and Yovel, and separating terumot and ma’asrot. We shall now explain.
Every contract must be dated, as it must be clear when the obligations it entails begin; pre-dated contracts are invalid. The first of Tishrei is the new year for documentary purposes (Rosh Ha-shana 8a). During the times of the Amora’im and Ge’onim, contracts were dated according to the Seleucid era (minyan shtarot). At the end of this era, Jews began dating documents from the world’s creation. This is the current practice for all contracts, including marriage and divorce documents.
Likewise, when the years are counted to determine Sabbatical and Jubilee years, the year begins in Tishrei (Rosh Ha-shana 8b). We are also commanded to separate terumot and ma’asrot from the produce of each year; one may not tithe from the produce of one year for the produce of another, as we read: “You shall set aside every year a tenth part of all the yield of your sowing that is brought from the field” (Devarim 14:22). Rosh Ha-shana is when the new year begins for this purpose (Rosh Ha-shana 12a).
. The year to which vegetables are assigned is determined by when they are picked, while the year to which grains are assigned is determined by when they have reached a third of their full growth. This also has implications for whether one must take ma’aser sheni (on years 1, 2, 4, and 5 of the Shemita cycle) or ma’aser ani (on years 3 and 6 of the Shemita cycle). While the year for vegetables and grains begins with Rosh Ha-shana, for fruit trees it begins with Tu Bi-Shevat. That is, on Tu Bi-Shevat, the age of the tree is incremented by one.
The parameters of the mitzva of orla are a bit different. We are commanded not to eat a tree’s fruits during its first three years, and the fruits of the fourth year are to be eaten in purity in Jerusalem. These years are counted from the first of Tishrei. For example, let us say that someone planted a tree on the fifteenth of Av. It takes two weeks for a tree to take root. After taking root, the tree is alive for an additional month before Rosh Ha-shana. These six weeks are the minimum amount of time necessary for the tree to be considered to have completed a year, so Rosh Ha-shana marks the beginning of its second year. Two more years must pass before its fruits are no longer deemed orla. However, since the new year for trees is Tu Bi-Shevat, one must wait from Rosh Ha-shana until Tu Bi-Shevat to eat the fruits. The fruits themselves are assigned to a year based on when they blossom (SA YD 294:4).
According to tradition, the first year of minyan shtarot corresponds to the year Alexander the Great became emperor (MT, Laws of Divorce 1:27), in 3449 from creation (312 BCE). This was the conventional dating system in the Seleucid Empire. It was in use throughout the latter part of the Second Temple era, and Jews used this system until medieval times. In fact, some Yemenite communities still record this date in ketubot. For more on the laws discussed in this section, see R. Shlomo Yosef Zevin, Ha-mo’adim Ba-halakha, Rosh Ha-shana, section 2.