import datetime
# 字符串日期+天数
>>> a = datetime.datetime.strptime("2021-08-28", "%Y-%m-%d") + datetime.timedelta(days=4)
datetime.datetime(2021, 9, 1, 0, 0)
# 格式化为字符串
>>> b = a.strftime("%Y-%m-%d")
'2021-09-01'
# 字符串日期相加减
>>> c = datetime.datetime.strptime("2021-09-03", "%Y-%m-%d") - datetime.datetime.strptime("2021-08-28", "%Y-%m-%d")
datetime.timedelta(6)
# 转化为整数
>>> c.days
6