'str' object has no attribute 'strftime'

def news_format_date(value):
    now_date = datetime.now().date()
    yesterday_date = (datetime.now() - timedelta(days=1)).date()

    if now_date.strftime('%Y-%m-%d') == value.strftime('%Y-%m-%d'):
        return 'Сьогодні, {}'.format(value.strftime('%H:%M'))
    if yesterday_date.strftime('%Y-%m-%d') == value.strftime('%Y-%m-%d'):
        return 'Вчора'
    return format_datetime(value, "dd MMMM y", locale='uk_UA')