Pokémon Chronology

Where each episode, movie and special sits in the timeline

Kyurem vs The Sword of Justice

Movie Fifteen was released at a time when Dawn was travelling with Ash in the anime, yet she does not appear in the film so some movement is required. Meloetta is also absent, yet during the time Meloetta follows Ash it makes frequent appearances especially when Ash is in peril, so the movie likely occurs outside of this arc.

Making the assumption that the movie occurs before the Meloetta arc, it must also occur prior to BW081 which sees the group board a boat until BW082. This appears to be best place, as geographically there doesn’t seem to be enough space between Twist Mountain and Icirrus City for Roshan City (setting of the movie) to exist in.

Source: Bulbapedia