Magical Girl Lyrical Nanoha A's Portable: The Gears of Destiny

The story of The Gears of Destiny starts around 3 months (March 0066) after the "Dark Fragments Incident" in The Battle of Aces.

Two sisters, Amitie and Kyrie, travel to this point in the timeline via time travel in an attempt to capture a powerful creature known as Unbreakable Darkness, or System U-D, which the remaining clones are attempting to revive.