Rabban Shimon ben Gamliel II is first attested as a youth at the fall of Betar in 135 CE and later as Nasi presiding at Usha from around 140 CE, but no source gives an actual birth or death year for him.
What would resolve this A datable document or inscription fixing his birth year, and a record of his death independent of his son Judah HaNasi's own, later-attested succession.
OpenTalmudic chronology, history of the Sanhedrin and the Hillelite patriarchateMichael Berenbaum and Fred Skolnik (eds.), Encyclopaedia Judaica, 2nd edition