Tu b'shvat is exactly forty days before the 25th of Adar. My chevrusa, Yaakov Raice told me that it is brought down that just as we have a concept of "forty days before the fetus begins to take form", we have a similar concept here. The 25th of Adar is the moment of Bereshis, according to one opinion in the Gemara in Rosh Hashana. It is when the world took its form. Forty days before is the day of Tu b'shvat, when the conception of the world occurs. This is true each year, as the world is renewed, and the trees begin to bud, showing this first thought, the מחשבה תחילה.
May we be zocheh to see the budding of the צמח דוד - the 'plant of David,' speedily and in our days. Amen.