Speaking of course from a strict biological perspective. The first cell of a new human that is different from the parental generation appears when the mother, destined to be, is still in the womb.
Therefore, the first biological cell of the current Emperor of Japan - Akihito - appeared most likely in 1902. Reason being, because his mother, Empress Kojun, was born in March 6, 1903 when Teddy Roosevelt was President.
Remember that after three to four months after fertilization, and if the embryo is female, the eggs of the next generation appear before birth.
What this means in the big picture, from a cellular point of view, Akihito was born during the Year of the Tiger. Also note the Emperor of Japan in 1902/1903 was Emperor Meiji.
Think about that!