import datetime

def pre_date(days=0, base_day=None):
    if base_day:
        base_day = base_day.strftime('%Y-%m-%d')
    else:
        base_day = datetime.datetime.now().strftime('%Y-%m-%d')

    return datetime.datetime.strptime(base_day,
                                      '%Y-%m-%d') - datetime.timedelta(days=days)
print(pre_date(-1))