PostgreSQL connections forced into UTC if setting.USE_TZ is True.

[chris-bradley]

DatabaseWrapper.ensure_timezone() in django/db/backends/postgresql/base.py forces the database to use self.timezone_name() as its timezone. On the base class, timezone_name() uses the following logic:

• Return settings.TIME_ZONE if USE_TZ is not truthy.
• Return self.settings_dict['TIME_ZONE'] if it is set.
• Return 'UTC' otherwise.

However, BaseDatabaseWrapper.check_settings() prevents 'TIME_ZONE' from being set if the supports_timezones feature is set on the backend. The database's timezone is thus always set to 'UTC' for databases such as PostgreSQL if USE_TZ is set to True.

This creates problems if the database does anything involving the current date, which may be off by one from the Django's date. For instance, consider the following setup:

• Model X points at a view.
• X has a field named age_in_days that points to a column defined as CURRENT_DATE - y, where y is a date column.
• The Django settings module has USE_TZ set to True and TIME_ZONE set to 'US/Eastern'.

X.age_in_days will then be one too high if viewed between 7:00 PM (8:00 PM if DST is in effect.) and midnight.

A solution that would not break backwards compatibility is to allow 'TIME_ZONE' to be set on a databases settings dictionary even if supports_timezones is set to True for the backend's features.

I have not checked if this is still an issue in later Django versions.

[Tim Graham]

Duplicate of #23524. Feel free to read the relevant threads on django-developers and open a new one if you still believe the behavior should change.