Planetarian/WMG

Planetarian is a prequel to Yokohama Kaidashi Kikou
Both take place After the End, both feature Ridiculously-Human Robots (that are female, no less), the main characters of both series are leaning to cope with loneliness as they wait for their owners, and both main characters develop attachments for those few humans that stick around. Given the circumstances of either work, it should be virtually seamless to fit Planetarian into the same timeline as Yokohama Kaidashi Kikou depicting the final stages of the war that lead to humanity's downfall. The Planetarian robot would be an early Alpha-type of robot, maybe the same one buried under the grave Kokone ran into before she entered the library.
 * The world in Yokohama Kaidashi Kikou is not in quite the desolate state as the one of Planetarian though. For one, there is no constant acid rain pouring down. It actually seems quite thriving, despite the rising sea-level, which makes it even more mystifying why humanity is becoming extinct.
 * Depending on the amount of time that passes between Planetarian and Yokohama Kaidashi Kikou, the acid rain simply could've been "cleaned" out of the system after the majority of the world's factories have fallen silent. likewise, a lot of the devastation (particularly to the human race) could've begun a slow repair process during that time. And finally, I don't buy the "humanity is actively going extinct" claim, even if it is "Word of God" especially considering that even in the second to last chapter we still see relatively thriving communities (Alpha visits one, where Maruko also happens to work at, remember?) and the last few chapters, including the epilogue, prove that humans are still actively reproducing.

Planetarian also belongs to the same timeline as ARIA
If Yokohama Kaidashi Kikou is in the same timeline ARIA (like the YKK WMG page suggests) and if Planetarian also in the same timeline as Yokohama Kaidashi Kikou, than the rules of logic state that, therefore, Planetarian is in the same timeline as ARIA as well.

Planetarian is in the same universe as WALL-E.
The Buy'N'Large Corporation was directly or indirectly responsible for the catalysts of the war. The war itself was the other reason as to the launching of the Axiom (though Shelby Forthright hid that reason from the unaware public because, you know, war. It'd crush their hopes.) and obviously, the wrecking of the Earth. However, BnL felt racist and decided to screw Japan (which was the only area plagued by the acid rain for some reason), thus why there are still some survivor remaining there. We can also assume that BnL helped in the development of Reverie and that at one point in his long life, WALL-E has taken her corpse into his stash of treasures.
 * Wow, that last image is actually pretty depressing.

Planetarian is in the same universe as Fallout 3
I'll let you draw your own conclusions on that one. Or, as someone from a certain Image Board said upon encountering an abandoned planetarium in Fallout 3, "I was expecting Reverie, but all I got were super-mutants!"
 * Then there's Fallout New Vegas, which has yet another planetarium in the REPCONN HQ, complete with a helper robot (albeit it's a Mr. Handy) and the antique Sentry Bot elsewhere, with four legs and machine guns. Hmmmm...

Planetarian is in the same universe as Ever 17
Specifically, it's the future that occurs after Takeshi's bad ending. The biological weapons that wiped out most of humanity were the result of Leiblich's perfection of Tief Blau. In fact, Leiblich's influence in most major governments was probably one of the major factors that caused humanity's self-destruction.
 * Furthermore, the robot body Sora gained in the true ending was still developed, leading to the line of robots that Reverie belonged to.

Planetarian takes place at the same time as Power Rangers RPM
Post-apocalyptic setting? Check. Killer bots that prowl every ruined city? Check. The endless rain? Venjix has the ability to bring down endless rain by messing with massive (country-wide) weather systems. And there's something in Reverie that prevents her from getting infected with the Venjix virus -- that's for Doctor K to discover.
 * Alphabet Soup, where Dr. K used to work at, had a bunch of other top-secret projects like Ridiculously-Human Robots, and Reverie is one of those robots. So when the virus struck, Dr. K's fellow scientists developed an antivirus, but only administered it on Reverie, as their other robots were already infected and were way past recovery due to the virus's fast-spreading nature.