Figured I'd show you something, thanks to this screenshot I got off Tumblr:
Now, this Original hat Pikachu is Lv1 and has the same moves like the Japanese event from earlier this year, but the Trainer ID No. is different to the Japanese one.
The Japanese events had ID No.'s that corresponded to the first airdate of that generation's series, as shown with the following ID No.'s, going in the Y/M/D format: 970401 (Original), 021121 (Hoenn), 060928 (Sinnoh), 100923 (Unova), 131017 (Kalos) and 161117 (Alola)
With this international Ash Hat Pikachu event, the ID No. instead corresponds to the first airdate of the English dub for that series, but also written in a different format from the Japanese ones, this time with the month coming first, then the day and then the year. Since the first Original series episode aired on September 8th, 1998 in the US (and therefore the dub) that means Original hat Pikachu has the ID No. 090898.
Based on this and the first official dubbed airdate for each generation (XY and SM's first two episodes airing as sneak peeks probably don't count), here's what will seem to be the ID No. for each hat afterwards:
Hoenn - 031503 (March 15th, 2003)
Sinnoh - 060407 (June 4th, 2007)
Unova - 021211 (February 12th, 2011)
Kalos - 011814 (January 18th, 2014)
Alola - 051217 (May 12th, 2017)