Oh lord vampire lore and someone had to mention Twilight... XD
In my opinion the timeline(s), if any, in the Pokémon world do not cause too much trouble. Even the apparent conflicts in one same canon, like game!canon, can be worked around with just a bit of care and without the need to dismiss or ignore something more official.
Take "dates", for example. If we go by game!canon, there's a specific calendar that includes the First Moon Landing (1969-07-20). But we're not given
any other particular date or reference that would allow us to pinpoint how much time ago was that, even when the games are supossed to occur "now" (after all, it is one who is being avatar'ed into the game). Yet take Gen II and Gen IV into consideration. These are stated a the beginning of D&P to take place pretty much at the same time, which by all purposes is "now". G/S was 10 years ago and the player character had a Game Boy (I think) at home, but surprise!! -- "at the same time"
ten years ago, the D/P player character had a DS and a plasma or flat TV. So that leaves us with two easy (to avoidentangling the plot) choices:
a) games!now is a future moving back and forth relative to "our" time (so that, for example, a Nintendo Virtual Mindcube 128 appears in the game).
b) timeline is fundamentally different and particular dates such as First Moon Landing are a coincidence or a out-of-universe convenient naming.
I'd personally go with a) because I love floating timleines, which are a very comfy plot device to help dealing with the different speeds of progress in a fic. For examples of prominent floating timelines (with regards to their own universes) one can watch TV shows such as Third Watch, 24, Boston Legal and Cold Case. I'm not sure if and how anime and manga make use of this device.
One can take into consideration that, as said a couple of posts above, industrial revolution could have happened before, tech development focus could have been moved to medical sciences during the period of time "our" world focused on the Space Race, and such. But the important thing is, once you pick an initial
diverging point, you are pretty much free to work up technology and dates any way you like (as long as they make internal sense, of course).
Heck, for all we know, the First Moon Landing in the Pokémon world could have been part of a scientific bet between two competing agents, and was abandoned after that and discarded in favor of Poké sciences. I mean, the "shuttle" image is at a museum in the same section as
Pokémon fossils.
Going strictly by the OP's question:
To elaborate, Canon says that this event occurred roughly 500 years ago - however, in order to have this event flow with what I intended to be the story, it'd be better if it occurred roughly 150-200 years ago.
Going by that info alone, 500 years ago the following events ocurred (source: Bulbapedia)
* The first Spiritomb is sealed in the Hallowed Tower.
* The Cianwood City Pharmacy is built.
Question:
why do you need the event 150~200 years ago?
If it is because there is a particular and convenient event or technology
in our universe that you need, you can simply stop here: mention (do not "offer" or "explain") an alternate timeline with thre pivotal points: the present of your story, the Spiritomb/Cianwood event that you state to have happened 150 years ago, and an undefinite time in the past where the timeline diverged from ours and the Pokémon world's. Period.
If it's because there is an
in-universe event or technology which you need to integrate (Bulbapedia says the Ecruteak Tower burning and the first Tournaments taking place), read below.
I'd say these two events pose no trouble to work around by floating the "general" Pokémon world timeline.
Spiritomb's sealing is a bit difficult to timefloat: you may require working around legends involving Spiritomb, the place's architecture and the people's awareness about it, what happened to
subsequent spiritombs and such, but is workable.
As for the Cianwood event that is pretty easily timeflotable because you can simply alter the past's rate of progress as long as you reach their "known" present (Gen II/IV?). It may be a trouble if Olivine City is featured too because you'll have to alter its past progress rate too because of the Lighthouse and the Whirl Islands, but I do not see more trouble.
Do not want to float? Choice B: you can simply use some creative license (which
is not the same as defiling canon!) to propose that the canon information originally provided refers to the events in an "incomplete" (eg.: 500 years ago the
first Cyanwood Pharmacy was built, but the one that exists
nowadays was built 150~200 years ago) or "inconcluse" (eg.: it is
believed that Spiritomb was sealed 500 years ago because that was around the time it disappeared) fashion. I can not see any of these conflicting with other elements of (well-known) game!canon as long as you remember to work around the local cultures a bit, in particular the Spiritomb as, if I recall correctly, is tied to the history of Solaceon Town.
I hope my suggestions are useful.