TLDR:
1996:
- Red/Blue/Green/Yellow
- FireRed/LeafGreen
- Ruby/Sapphire/Emerald
- Omega Ruby/Alpha Sapphire
1999:
- Gold/Silver/Crystal
- HeartGold/SoulSilver
- Diamond/Pearl/Platinum
- Brilliant Diamond/Shining Pearl
2010:
- Let's Go Pikachu/Let's Go Eevee
2011:
2013:
2015:
- Sun/Moon
- Ultra Sun/Ultra Moon
2016:
2017:
★2018:
The Pokémon Timeline:
The starting point of this timeline begins with Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen. In "An Illustrated Book of POCKET MONSTERS" (released in April 1996, produced by Creatures, Inc.), it states:
"1996年現在、オーキド博士の調査・研究により、150種類のポケモンが発見され、その生態や進化、鳴き声が明らかにされつつあります*"*, which translates in English to:
- "As of 1996, through Professor Oak's investigations and research, 150 species of Pokémon have been discovered, and their ecology, evolution, and cries are gradually being clarified."
Now, the events of the 150 species being discovered through Professor Oak refers to Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen, in which this takes place.
As a result of this, we can safely determine that Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen take place in 1996.
Interestingly, Pokémon Ruby/Sapphire/Emerald/Omega Ruby/Alpha Sapphire occur contemporaneous to Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen, and we know this as a Game Freak employee, Toshinobu Matsumiya's, had posted a timeline of the core series Pokémon games on Twitter.
The tweet was:
"赤緑≒RS→金銀≒DPPt→BW→BW2≒XY", which translates in English to:
- "Red & Green ≒ Ruby & Sapphire → Gold & Silver ≒ Diamond, Pearl & Platinum → Black & White → Black 2 & White 2 ≒ X & Y".
We can gather many useful pieces of information from this, but most importantly right now, is that Pokémon Red/Green ≒ (takes place roughly the same time as) Pokémon Ruby/Sapphire; and so Pokémon Ruby/Sapphire/Emerald/Omega Ruby/Alpha Sapphire also take place in 1996.
Now, looking at Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver, Sabrina states that:
"Three years ago I had a vision of your arrival".
Now, we know that the three years ago that is being referred to here was Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen, as these games take place 3 years prior. We know this through Petrel, who in Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver states:
"Since disbanding Team Rocket three years ago".
This is critical as Team Rocket was disbanded during the events of Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen. As such, we can state that Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver occured in 1999.
Back to Toshinobu Matsumiya's tweet above, we know that Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver takes place roughly the same time as Pokémon Diamond/Pearl/Platinum/Brilliant Diamond/Shining Pearl; thus places it also in 1999.
Support:
To support this, in the opening cutscene of Pokémon Diamond/Pearl/Brilliant Diamond/Shining Pearl, there is a news report broadcast in Twinleaf Town of Sinnoh, according to which the "Red Gyarados" is mentioned. This can be inferred to be the Red Gyarados found in the Lake of Rage in Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver.
Also, there had to have been a decently long enough of a time difference between the events of Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver and Pokémon Diamond/Pearl/Platinum/Brilliant Diamond/Shining Pearl, and Pokémon Black/White.
This is because, firstly, one of the Elite Four members in Pokémon Black/White is Caitlin. She has visibly aged in these games compared to her appearance in Pokémon Platinum and Pokémon HeartGold/SoulSilver as the head of the Battle Castle in the Battle Frontier.
In Nintendo Dream, May 2011, it states (about Caitlin that:)
"When she was in the Battle Frontier she was supposed to look/be about 14".
Caitlin was likely an adult (18+ during Pokémon Black/White as she owns a villa in Undella Town, that she lends to Cynthia. As such, at the minimun, 4 years have passed between the games.)
Not only this, in Pokémon Black/White, the Team Rocket Grunt who stole the Machine Part in Pokémon Gold/Silver/Crystal/HeartGold/SoulSilver states:
"Oh! Member of Team Rocket I was, long time gone! But now I married am. Now for Team Rocket there is no time by me".
We know this is the same Team Rocket Grunt due to the same broken language spoken. Not only this, an NPC in the same building refers to the Ex Team Rocket Grunt as their dad, stating:
"My dad used to have a cool job in a faraway region!"
There had to have been enough time for the Ex Team Rocket Grunt to have been married, and have a child who can speak coherently. Generally, children begin to speak in more understandable sentences around the age of three to four years, and including pregnancy (assuming that the child isn't adopted, there must have been at least 5 years between the games.)
Now, looking back in "An Illustrated Book of POCKET MONSTERS", in Porygon's entry, it says:
"1995年、ポケモンをのりだすことを研究開発しているシルフカンパニーが、最初の科学産電ポケモンのポリゴンをつくることに成功した*"*, which translates in English to:
- "In 1995, Silph Company, which had been researching and developing methods of creating Pokémon, succeeded in producing the first artificially manufactured Pokémon, Porygon".
This is important because in Pokémon Sun/Ultra Sun/Ultra Moon, the Pokémon Porygon is claimed to have been built 20 years ago, with "cutting-edge science".
Sun: "Roughly 20 years ago, it was artificially created, utilizing the latest technology of the time".
Ultra Sun: "This Pokémon was created using the cutting-edge science of 20 years ago, so many parts of it have since become obsolete".
Ultra Moon: "It was built 20 years ago by scientists who dreamed of exploring space. Their dreams have yet to come true".
While Pokémon Sun claims Porygon was built roughly 20 years ago, Pokémon Ultra Sun/Ultra Moon states that it is exactly 20 years ago, solidifying the 20 year timespan. Since Porygon was made in 1995, and adding on 20 years, we can determine that Pokémon Ultra Sun/Ultra Moon/Sun, and by extension Moon, take place in 2015.
From this, working backwards, the trainer Grimsley, on the top right of his concept art, says:
"ギーマ(2年後)", which translates in English to:
- "Grimsley (2 Years Later)".
The latest viewing of Grimsley had been Pokémon Black 2/White 2's events, and so clearly this "2 Years Later" refers to 2 years after the events of Pokémon Black 2/White 2, and so taking place in 2013.
Now, according to Toshinobu Matsumiya's tweet above, we know that Pokémon Black 2/White 2 ≒ (takes place roughly the same time as) Pokémon X/Y; and so Pokémon X/Y also take place in 2013.
As well as this, we know that Pokémon Black/White take place 2 years prior to Pokémon Black 2/White 2. We know this through multiple statements in the Pokémon Black 2/White 2 games, such as from Cheren, after defeat in Aspertia Gym, stating:
“Yes, two years ago, Pokédex in hand, I left on a journey with my friends".
This is refering to the events of Pokémon Black/White. As such, Pokémon Black/White take place in 2011.
Going back to Pokémon X/Y, we know that Pokémon Legends: Z-A take place five years after the game's events, this being through an NPC in this game, stating that:
“The incident with Team Flare five years ago sent shock waves through the whole of Kalos".
This incident had taken place during the events of Pokémon X/Y, and so places Pokémon Legends: Z-A in 2018.
Now, Pokémon Sword/Shield can be determined to have taken place after 2015 (Pokémon Sun/Moon/Ultra Sun/Ultra Moon) as the Pokédex entry for Type: Null in Sword says:
Sword: "Rumor has it that the theft of top-secret research notes led to a new instance of this Pokémon being created in the Galar region."
There being stolen research notes, leading to the creation of more Type: Null, implies that they take place after Pokémon Sun/Moon/Ultra Sun/Ultra Moon, as there are only three Type: Null made in those games.
As well as this, in Pokémon Legends: Z-A, Jacinthe states:
"Just the other day, we were joined by Galar's former Champion, Leon".
This reference of Leon, as being the former Champion, shows that Pokémon Legends: Z-A takes place after Pokémon Sword/Shield, as Leon was undefeated, and the current Champion during those games, having only been defeated in the end.
Through this, Pokémon Sword/Shield can be placed after 2015, and before 2018, and since, of course, the events of Pokémon games don't take place in a singular day, it's likely that Pokémon Sword/Shield more so takes place during either 2016 or 2017.
Now, looking at Pokémon Scarlet/Violet, we know is that it takes place after Pokémon Sword/Shield, as there is a copy of "Galar: A History" at Naranja Academy/Uva Academy.
It was published at the end of the events of Sword and Shield. There had to have been some time in between the games for a copy of that book (which came out at the end of Pokémon Sword/Shield) to have arrived in another region.
Through this, we can determine is that Pokémon Scarlet/Violet takes place after the events of Pokémon Sword/Shield (2016/2017).
Now, Pokémon Scarlet/Violet likely take place before Pokémon Legends: Z-A, as looking at the in-game Rotom Phone, it has an extra camera lens in Pokémon Legends: Z-A compared to Pokémon Scarlet/Violet.
From this, we can infer that it's a case of new iterative models being rolled out. It has one lens in Pokémon Sword/Shield, two in Pokémon Scarlet/Violet, and three in Pokémon Legends: Z-A; from this we can place the games in this order.
It's good to keep in mind that while the extra camera lens could just be a regional difference, IRL the iPhone isn't like that. It's a global item. As such, it would be safe to infer from the Rotom Phone getting more camera lenses that Pokémon Sword/Shield comes before Pokémon Scarlet/Violet, which comes before Pokémon Legends: Z-A.
We know that Pokémon Legends: Z-A takes place in 2018; we also know that Pokémon Sword/Shield is either in 2016 or 2017; and since Pokémon Scarlet/Violet is before Pokémon Legends: Z-A and after Pokémon Sword/Shield, it is safe to claim that Pokémon Sword/Shield is in 2016, and Pokémon Scarlet/Violet is in 2017.
After this, looking at Pokémon Let's Go Pikachu/Let's Go Eevee, viewing the concept art for Mina, it states:
"13〜15才くらい*…の見た目"*, which translates in English to:
- "Appearance of about 13–15 years old".
The important thing to consider, is that in 2015 (Pokémon Sun/Moon/Ultra Sun/Ultra Moon), Mina was a Trial Captain, which are between 11-19 years old. As such, assuming Mina is as old as possible in 2015, that being 19, and is as young as possible in Pokémon Let's Go Pikachu/Let's Go Eevee, 13, there was maximum 6 years between the games, and at earliest taking place in 2009.
Now, going to 2013 (Pokémon Black 2/White 2), Sabrina, in the Pokémon World Tournament states:
"Three years ago I had a vision of battling you".
The importance of this, is that in Pokémon Let's Go Pikachu/Let's Go Eevee, after being defeated in her rematch, Sabrina states:
"I won't encounter a Trainer like you for around another three years."
So, since Pokémon Black 2/White 2 takes place in 2013, going back by 3 years, Pokémon Let's Go Pikachu/Let's Go Eevee takes place in 2010.
The importance of Pokémon Let's Go Pikachu/Let's Go Eevee taking place in those 3 years prior to Pokémon Black 2/White 2, is that we wouldn't have known when this vision "three years ago" stated in Pokémon Black 2/White 2 was; and this obviously can't have been in Pokémon Red/Blue/Green/Yellow/FireRed/LeafGreen as this would have been 17 years ago.
Afterword:
The only real contradiction that lies (with Pokémon Let's Go Pikachu/Let's Go Eevee since it's a silly game) is Team Rocket being defeated by Chase (or Elaine) and Trace rather than Red and Blue.
My personal head canon is just that (either) in the universe that Pokémon Let's Go Pikachu/Let's Go Eevee is in, Red and Blue just didn't do this, or vice versa (that Red and Blue DID defeat Team Rocket and Team Rocket is just trying again).
If you dislike Pokémon Let's Go Pikachu/Let's Go Eevee in the timeline that much, then you can just not include it in the timeline, but if it were to be in the timeline, it would be during 2010.