“Quantumania” kicks off with a brief flashback that takes place in the Quantum Realm. Janet is hiding out for her own safety in what she later calls Subatomica. She sees a ship crash in the distance and on her way to approach it, she deftly fends off some dangerous-looking creatures. But this isn’t Hank’s voyage to fetch her from the hidden universe in “Ant-Man and the Wasp,” which led to their reunion. Instead, moments later, a distraught Kang reaches out for help.
A refresher: Janet got trapped in the Quantum Realm in approximately 1987, when she turned off her regulator and went subatomic to stop a missile from killing thousands. She put an antennae in Scott’s head during his momentary visit to the Quantum Realm in 2015 and was rescued by Hank in 2018, meaning she spent about 31 years there.
While the present timeline within “Quantumania,” in which Cassie’s device sucks the Pyms and Langs into the subatomic universe, takes place in 2025, this flashback takes place sometime between 1987 and 2018… probably much closer to her arrival than her departure. After their falling out, Kang had time to conquer the native population and build his citadel, but then again, time works a little differently for him, so who knows how long that would take? Generally speaking, the film’s main action runs concurrent to most of Phase 4, starting with “Moon Knight,” which is also set in 2025.