The dating of events before the rising of the Sun is difficult, and that is especially true for events as far back in history as these. The dates shown here are based on those given in The Annals of Aman in volume 10 of The History of Middle-earth, but the dating there is not exact, and the conversion of Valian Years into solar years introduces another level of uncertainty.
Specifically, those annals date the raising of Illuin as the 1,500th Valian Year of the Valar in Arda (or shortly afterward) and its destruction as the Valian Year 3,450 on the same system. One Valian Year is equivalent to approximately 9.58 solar years (at least on the conversion system used here), producing the figures shown above.
Though these dates are approximate, they give some idea of the vast scale of time involved: Illuin stood for more than six times the length of the entire Third Age before its downfall.