This arc is what is called "filler." It is, as the name states, an arc or story line created by the animation team to "fill" time whilst waiting for the manga to progress further into the story so that the animation team can continue to produce the story. It is not canon.
However, to answer the question that you posed, the arc takes place, chronologically speaking, after the Sousuke Aisen arc, but before the Fullbringer arc.
Additionally, the "replacements" arc, in which a treasonous member of the research and development squad (12) creates mod-soul copies of the shinigami, is also a filler arc, and is non-canon.