Not exactly. Many people go on a theory of alternate dimensions. The first consisting of Final Fantasy Tactics, Vagrant Story, and Final Fantasy Tactics Advance. In tactics Advanced it is theorized that while Marche, Ritz, and Mewt returned to their world, the fantasy world they dreamed up lived on and this fictional Ivalice is then the setting for Final Fantasy XII, and any following games in the "Ivalice Alliance" (bar Final Fantasy Tactics: War of the Lions as this is a remake, but then they do Balthier from XII, and Luso from Tactics A2), None of this is comfirmable though until noted by Square.
The Ultimania guide does confirm that the Ivalice of FFXII is the Ivalice of FFT and FFTA2, and that there is one timeline. The real confusion is in the geography, but it's possible that the Cataclism that occured after Ajora's death is what changed the landscape.