RE: But what if it's coming from the day side of the Earth?
It doesn't need to be coming form the "day side".
I translated the Mayan texts myself. They are actually surprisingly specific on this subject. They clearly state that Irnbru was ejected from the galactic nucleus of Andromeda at a velocity of 0·99573528C
Those malevolent Mayan munchkins received a tip-off about our impending apocalypse in the form of a radio message sent by (now extinct) inhabitants of a planet towards the edge the Andromeda system. The archaic Andromedonians witnessed the evil ejaculation as it occurred - some 2·5 million years ago. They calculated the terrible trajectory of the ominous orb and its inevitable impact with our paltry planet, and, seeing that our beautiful blue ball was supremely suitable for sustaining sentient species, they were kind enough to send a warning message our way. The message, travelling at ~C, soon overtook the slightly slower moving "planet" and arrived at earth on the morning of 11th May 6022BC. It stated the precise interval between message arrival and impact. Hence the "calendar".
So, you see, it's all true. And no... We certainly can't see it coming. I don't know how those NASA "boffins" can have missed something so obvious. Perhaps they got their translation from Google.