The primary use case is to interact with a third-party database (not
primarily managed by Django) that doesn't support time zones and where
datetimes are stored in local time when USE_TZ is True.
Configuring a PostgreSQL database with the TIME_ZONE option while USE_TZ
is False used to result in silent data corruption. Now this is an error.
As suggested by Anssi. This has the slightly strange side effect of
passing the expression to Expression.convert_value has the expression
passed back to it, but it allows more complex patterns of expressions.
Aymeric Augustin
Jon Dufresne
Marc Tamlyn
Tim Graham