Tu BiShevat and Orlah in and out of Israel

Answer

Tu BiShevat is the New Year for trees, and is significant for terumot and ma'aserot as well as orlah. The Torah states (Vayikra 19:23): "When you enter the land and plant any tree for food, you shall regard its fruit as forbidden. Three years it shall be forbidden for you, not to be eaten."

In the first three years from when the tree is planted, its fruits are called orlah, and we cannot eat or benefit from them.

Since Tu BiShevat, 15 Shevat, is the New Year for trees and not 1 Tishrei, fruit that began to develop (chanatah) after 15 Shevat of the fourth year from when the tree is planted are no longer orlah.

Orlah outside of Israel

Vaday orlah, fruit that is certainly within its first three years, is forbidden even outside the land of Israel (as a halachah leMoshe miSinai). Even though orlah is a mitzvah dependent on the Land of Israel, it also applies abroad.

So if you know it is orlah (it's your tree, for instance), it is forbidden. However, if you are not sure whether a fruit came from an orlah tree, it is permitted to eat (even if likely orlah, but you are not 100% certain) outside Israel.

In Israel, in contrast, when there is a doubt about orlah, it is forbidden to eat the fruit.