Doctor Strange and the Masters of the Mystic Arts can still travel in time - even without the Time Stone. The Masters of the Mystic Arts have kept the Time Stone for centuries at least, if not longer. They considered the Time Stone to be their greatest weapon, with both the Ancient One and Doctor Strange using it to explore future timelines and work out how to engineer events to prevent cosmic catastrophes.
The sorcerers’ long experience with the Time Stone means that they understand time itself better than anyone else in the MCU. Indeed, that’s why Avengers: Endgame had the Ancient One provide a Time Travel lesson to the Hulk, with her exposition intended to clue viewers in to the loose “rules” of time travel in the MCU. Some of the secrets of temporal manipulation are kept in the Book of Cagliostro, an ancient tome that taught Doctor Strange how to use the Time Stone. Most viewers assume that all this knowledge and experience is useless now the Time Stone has been destroyed, because the only way to travel through time now appears to be via the Quantum Realm.
But Doctor Strange, at least, may still have the ability to travel through time - even without his Infinity Stone. It’s important to remember that the Masters of the Mystic Arts understand the Multiverse far better than Hank Pym, Scott Lang, and Tony Stark. The Doctor Strange movie included a so-called “Magical Mystery Tour” in which Doctor Strange’s astral form was cast through countless dimensions. He even visited the Quantum Realm, proving that the Masters of the Mystic Arts have at least a basic familiarity with this mysterious plane of existence. What’s more, if the sorcerers know of the Quantum Realm, there’s absolutely no reason they couldn’t reach it.
Doctor Strange introduced the concept of Sling Rings, mystical devices used by the Masters of the Mystic Arts to focus their energy. Using the Sling Ring, a sorcerer can open a portal to anywhere they can envision with their mind. This doesn’t just mean they can travel from Wakanda to the United States, or from Titan to Earth; it means they can explore other dimensions as well. As Mordo explained to Doctor Strange when he was training him in the use of the Sling Ring, “they allow us to travel throughout the Multiverse.” Doctor Strange should be able to enter the Quantum Realm itself by use of the Sling Ring, and then exit at any other point in time and space.
Of course, the Masters of the Mystic Arts may have never used the Quantum Realm to travel through time. They may have never actually needed to time travel; and if they did so, they likely used the Time Stone, and never tried any alternatives. But Doctor Strange learned it could be done in Avengers: Infinity War, when he saw 14 million alternate futures and learned the only one where the heroes triumphed involved time travel through the Quantum Realm. He not only has the means - he is also the one Master of the Mystic Arts who definitely has the knowledge.
All this confirms that Doctor Strange is one of the MCU’s most powerful heroes. The Avengers need to build time platforms and quantum tunnels; but in order to travel through time, Doctor Strange just needs a basic piece of his kit. Given the Doctor Strange sequel is titled Doctor Strange In The Multiverse Of Madness, it’s entirely possible the one-day Sorcerer Supreme’s “Multiverse of Madness” involves manipulating time and the Quantum Realm.
More: Did Thanos Start Doctor Strange’s Multiverse Madness In Endgame?
