django date format 'dd-mm-yyyy'

In Sevenearths answer there's the comment

Why USE_L10N = True can't just do this I don't know!

The documentation says that USE_L10N = True is the default setting, but to get UK dates to work the LANGUAGE_CODE must also be changed from its default setting of en-us to en-GB.

Once changed, date fields automatically accept the formats "dd/mm/yyyy" and "yyyy-mm-dd" (and possibly others, I've not tested them all) without needing to set input_formats = settings.DATE_INPUT_FORMATS in every form date field.

DATE_INPUT_FORMATS in settings.py has no effect with USE_I18N = True. This is because django will load specific format for active locale. Django ships with format definitions for many different locales.

You can override django default format definitions:

mysite/
    formats/
        __init__.py
        en/
            __init__.py
            formats.py

As described in django documentation: Creating custom formats file

The problem turned out to be that I needed both the ISO and UK date formats in the settings.py file like so:

DATE_INPUT_FORMATS = ('%d-%m-%Y','%Y-%m-%d')

and then the forms.py adjusted to the following:

class ClientDetailsForm(ModelForm):
    date_of_birth = DateField(input_formats=settings.DATE_INPUT_FORMATS)
    class Meta:
        model = ClientDetails

Why USE_L10N = True can't just do this I don't know!

[Thanks to this and this]