Technically we already got Time Mage in the form of Astrologian as that Job already manipulates time and space anyway (albeit using cards, but it's still influencing the stars, planets and the passage of time). Also, one of Time Magic's big spells in past FF games, Haste, Yoshi specifically mentioned ages ago (back during the Beta Test in fact) will never be implemented as he felt spells that sped up time casting and attacking made the game too easy, so he decided early on not to implement such spells. Now, he may have changed his stance since on that, but as far as I'm aware he hasn't.

Either way, there is no need for Time Mage as it's place is already taken with AST I'm sorry to say.