The phrase "it was at that time" is correct and can be used in written English
For example: "They had been dating for two years when it was at that time that he proposed to her."
It was at that time that eastern Africa assumed its present topographic character.
It was at that time that the animal was recognized as its own species and named in Grévy's honor.
It was at that time that they became separated.
But the market is vastly different now to what it was at that time.
It was at that time the residence of the Sāsānian governor.
It was at that time that he adoped the name Ne Win.
It was at that time that she also met and married Mr. Barstow.
Paradoxically, it was at that time the plaza began springing back to life.
It was at that time that federal investigators made their first unsuccessful efforts to meet with Rodriguez.
It was, at that time, considered a "cool" firm in which employees shared in the success; many became incredibly wealthy.
It was at that time that Gleizes began to explore religion and the conflicts between a life of faith and a life of art.
