Date format issue in Django 5.1.6

[Li,Qianqian]

In Django 5.1.6, when setting USE_L10N = False and DATETIME_FORMAT = 'Y-m-d H:i:s', the expectation is that DateTimeField in the Admin interface displays as '2024-04-15 23:33:00'. However, it displays as 'April 15, 2024, 11:33 p.m.' (default en-us format). This worked correctly in previous versions (e.g., 5.0.x or 4.x).

Steps to reproduce:

1. Configure settings.py as follows: LANGUAGE_CODE = 'en-us' TIME_ZONE = 'Asia/Shanghai' USE_I18N = True USE_TZ = True USE_L10N = False DATETIME_FORMAT = 'Y-m-d H:i:s'
2. Create a model with a DateTimeField.
3. View the field in the Admin interface; the format does not follow DATETIME_FORMAT.

Expected behavior: Displays '2024-04-15 23:33:00'.
Actual behavior: Displays 'April 15, 2024, 11:33 p.m.'.

[Sarah Boyce]

This is also the case for Django 5.0.x, as it has been the case since USE_L10N was removed in Django 5.0
I believe this is a duplicate of #35990, in that the overridden date formatting settings are ignored when USE_I18N is True.

This behavior is by design and well documented, for example ​https://docs.djangoproject.com/en/5.1/ref/settings/#datetime-format

Note that the locale-dictated format has higher precedence and will be applied instead.

Please start a discussion on the ​Django Forum if you believe this behavior should change.