Rosh Hashanah 10 - The New Year of trees
The years of a tree are also counted from the first of Tishrei. Why is it important to know how old a tree is? - Because for the first three years the fruit of the tree are called "orlah" and cannot be eaten, so we need to know when to count the first year. As it turns out, if the tree is planted at least 30 days before Rosh Hashanah, this is already considered the first full year of its life. Even so, after the the third Rosh Hashanah, the fruit are not permitted immediately, but only after the fifteenth of Shvat, also known as "Tu B'Shvat."
From the rule above we see that thirty days in a year a considered a full year. However, this is only the opinion of Rabbi Elazar. By contrast, Rabbi Meir says that even one day in a year is already considered a full year. If so, how does Rabbi Meir explains the rule of thirty days for orlah cited above? He says that to take root the tree indeed needs thirty days, but if not for that, even one day in a year would make it a year old.
From here we can see that these teachers disagree on the months in which the world was created. How? - We will see this on the next page.
Art: Landscape with Oak Trees By Caspar David Friedrich