Generally, a landlord cannot backdate a rent increase to cover a period that has already passed. A rent review must follow the required notice rules, including giving at least 90 days’ notice before the new rent takes effect, and any applicable limits must also be observed. You should not treat a demand for backdated rent as automatically valid; ask the landlord to explain the legal basis and check the current RTB guidance. If the issue cannot be resolved, the RTB dispute resolution process can decide it based on the tenancy’s facts, or you can seek advice from a solicitor.