为什么我不能将datetime.date子类化？

from datetime import datetime

class MyDateTime(datetime):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

    def add_days(self, days):
        self += datetime.timedelta(days=days)

    def subtract_days(self, days):
        self -= datetime.timedelta(days=days)

d = MyDateTime(2022, 1, 1)
d.add_days(5)
print(d)  # 输出：2022-01-06 00:00:00
d.subtract_days(10)
print(d)  # 输出：2021-12-22 00:00:00