With nationwide my mortgage deal was agreed in Oct to start in Dec. Runs for the full 5 years to end of Nov.Some products are a fixed term from completion and some are fixed term from a month-end. We have a '5 year' fix which runs until August 2026 but we didn't complete until November 2021 (bit annoying, as our rate is 1.09% so we miss out on 3 months of that!).