They took the concepts of Time Mage in most FF games and the Corsair and combined them into a single class making the Astrologian this time/space diviner. I mean it sucks that they are not called a Time Mage as in tradition, but it makes sense for the lore on why they exist and what they do. Hell even the experts in Ishgardian astrology like Forlemort in the First Dicasterial Observatorium of Aetherial and Astrological Phenomena wear Time Mage hats. Ability wise, Astrologians they have many of the abilities that a Time Mage should have, but they are experts in fate as well as time/space.