Time Mages are also more than simply mages that influence time and use gravity spells. To wit:
While not so at first, spells like Meteor fell into the dominion of Time Mages. This means spells like Malefic actually resemble Time Magic as it technically attacks with stellar power.
If you take into account the Time Magic ability Chronos then you tie in both Time Dilation and Celestial Opposition in the fact that they increase the duration of beneficial effects they placed on themself/allies.
While not technically the same thing, The Arrow card is as close to a Haste spell as we are probably ever gonna get.
It is true AST lacks a lot that Time Mage has, but at the same time what more is there to give it? Blink and Reflect might be a little OP in this game as an example. Also Meteor is already taken by Limit Break 2 for Caster DPS, so that's out. Comet would be a lot like Malefic so no reason to add that. Would be nice if we got Slow but then we'd need Haste as well to make it more fitting, which won't happen since we have The Arrow. What AST lacks though Square Enix decided to fill with another job altogether lol but I digress.
Also if you think about it, don't the stars and fortune telling also relate to time? After all, you are reading the Past, Present, and Future.