Breaking Changes
datetime fields now use UTCDateTime, a custom type based on DateTime(timezone=True). It requires aware datetime parameters, normalizes them to UTC, and returns aware UTC values from database reads, including on SQLite and MySQL/MariaDB. Pydantic's AwareDatetime uses the same type, while NaiveDatetime explicitly selects DateTime(timezone=False). Plain datetime validation still accepts both aware and naive values, but naive database parameters now raise an error.
Existing databases are not changed automatically. To retain naive storage, use NaiveDatetime or an explicit Field(sa_type=DateTime(timezone=False)). Explicit SQLAlchemy types retain their existing behavior. PostgreSQL needs a column migration that explicitly specifies the timezone of existing values. SQLite and MySQL/MariaDB keep their column definitions, but existing non-UTC values need a data migration before adopting the new type. Update datetime producers, query parameters, comparisons, and any database defaults to follow the new UTC convention.
Read Datetimes and Timezones: Upgrade Existing Applications before upgrading.