def valid_up_to_column(self):
    now = datetime.datetime.now(datetime.timezone.utc)
    delta = (now - self.valid_up_to).seconds

    if delta > 900:
        colour = '#FF0000'  # red
    else:
        return self.valid_up_to
    return format_html('<span style="background-color: {}">{}</span>', colour, self.valid_up_to)

from django.utils.dateformat import format

datetime_str = format(self.valid_up_to, settings.DATETIME_FORMAT)

from django.utils.formats import localize

formatted_value = localize(value)

def localize(value, use_l10n=None):
...
    elif isinstance(value, datetime.datetime):
        return date_format(value, 'DATETIME_FORMAT', use_l10n=use_l10n)
    elif isinstance(value, datetime.date):
        return date_format(value, use_l10n=use_l10n)
    elif isinstance(value, datetime.time):
        return time_format(value, 'TIME_FORMAT', use_l10n=use_l10n)
...