In GSC, Yellow's birthday passed, but Gold's didn't, so the arc would be between April and July, no? More specifically, Blue's birthday didn't pass either, so April or May. If Serebii's Dex Holder page is accurate with the ages per arc, so if Crystal is really 13 in Emerald, then the GSC arc would be between April 30 and May 31.
Basically copying and pasting from the age difference chart I posted a while back that's been lost in time:
Start from Emerald. July 1 ~ July 7. Based on the ages given there, the actual age/birth year differences can be obtained.
Blue: 17
Red, Green: 16
Yellow: 14
Gold, Silver, Crystal: 13
Ruby: 12
Sapphire, Emerald: 11
Blue, Yellow, Crystal, Ruby, and Emerald had their birthdays pass. So the actual ages at the end of the year:
Red, Green, Blue: 17
Yellow, Gold, Silver: 14
Crystal: 13
Ruby, Sapphire: 12
Emerald: 11
The logic being: If you know someone who was born on the same year as you, but only a few months earlier, would you really say that that person is strictly older than you?
So, if Gold and Silver are older than Crystal, yet all three are 11 in GSC, then Gold and Silver's birthdays did not pass, but Crystal's did (so earliest start date of GSC is April 30). Blue and Crystal are 4 years apart. But if Blue is 14 in GSC and Crystal is 11, then Blue's birthday did not pass. Thus GSC can't be later than May 31, or Blue would be 15.
In RGB, Red, Green, and Blue are all 11. So either no one's birthday has passed, or everyone's did. So the arc is either before June 1, or after November 22. Since Yellow is 2 years after RGB, it's likely that no one's birthday has passed.
In Yellow, Yellow is 11, Red, Green, and Blue are 13. So their birthdays haven't passed, but Yellow's did. So the arc is between March 3 and June 1. At the end of this year, Red, Green, and Blue are 14, so if Yellow really is 2 years after RGB, then Red, Green, and Blue would've turned 12 at the end of that arc.
Add in the Sinnoh trio. The arc starts on September 28 ish (whenever DP was released in Japan), and passes October 27. So Diamond, Pearl, and Platinum are all 12. Diamond and Pearl saw a certain comedy act when they were 4, and that same act was seen in Johto shortly after the GSC arc, and given how any comedy act doesn't (and shouldn't) repeat for years, it's safe to assume that DP is 8 years before GSC, or 3 years before HGSS. So when Gold and Silver are 17, Diamond, Pearl, and Platinum are 9.
Red, Green, Blue: variable X
Yellow, Gold, Silver: X - 3
Crystal: X - 4
Ruby, Sapphire: X - 5
Emerald: X - 6
Diamond, Pearl, Platinum: X - 11
BW arc is approx 3 years after Pt. But ages aren't certain because we don't know Black's birthday and when exactly the BW arc takes place. And White is a variable. So BW and onwards are unknown. Assumptions have to be made.
Black, White: var
Rakutsu, Faitsu: var - 4 (likely var - 3)
X, Y: ???