Complex sentence: It was founded on October 30,1998 (Maradona's 38th birthday) in the city of Rosario, but it wasn't until 2001 that they had their first gathering.
Split into simpler sentences: It was founded on October 30,1998 (Maradona's 38th birthday) in the city of Rosario. But it wasn't until the year of 2001 that they had their first gathering.