Sorry, I couldn't resist.
Actually, if we held like Beit Shammai, Tu Bishvat would have been marked yesterday, on the first of Shvat. We accept Beit Hillel's view and mark it on the fifteenth.
Odd that we make such a big deal over the start of the tax accounting year for tree-borne fruits. Why don't we do the same for the first of Elul - the new year for animal tithes? We could eat beef jerky!