If Planet Y orbits the Sun but does not rotate on its axis at all, how long would a day be on Planet Y?

A day which in this case is longer than a year

At that time, the Universe was made out of about 92% hydrogen atoms and 8% helium atoms by number (or about 75-76% hydrogen and 24-25% helium by mass), with trace amounts of lithium and beryllium, but not much else. But you might wonder how it got to have exactly that ratio? After all, it didn't have to be that way; if the Universe was hot and dense enough to undergo nuclear fusion early on, why did it only fuse atoms up to helium, and why didn't more of the Universe become helium than it did?

To find the answer, we need to go way back in time. Not just to the first few hundred thousand years of the Universe, when it was making the first atoms, nor even to the first years, days, or hours. No, we need to go back to when the temperatures were so high, when the Universe was so hot, that not only could atomic nuclei not form (for they'd be immediately be blasted apart), but to a time when the Universe was so hot that the Universe was filled with nearly equal amount of matter-and-antimatter, when it was just a fraction of a second old!