Today, T”U BISHBAT, is the fifteenth day (in Hebrew the letter ט/T=9 and the letter ו/U=6. T+U=15 ) of the month of Shebat, which is the new year, Rosh haShana, of the trees. In the fifteenth of Shebat the trees are considered one year older. Halakhically speaking, the age of a tree does not increase on the date it was planted (the tree’s “birthday”); all trees become one year older on the same day, T”U Bishbat. This in an important factor for many Mitsvot connected to agriculture, for example, ‘orla: the prohibition to eat the fruits of a new tree during the first three years. For this and other Mitsvot one needs to know the age of the tree (see more details of the application of T”U Bishbat in the Mitsvot here). By the way, unlike most Mitsvot related to trees and plants, which pertain only to Israel, ‘orla is a Mitsva that applies not only in Israel but outside of Israel as well.