Python问题，在两个字典中判断key是否相同，若相同就把value值相减，value值是时间，时间格式：2021-06-24 07:30:36.0

如题
Python问题，在两个字典中判断key是否相同，若相同就把value值相减，value值是时间，时间格式：2021-06-24 07:30:36.0

运行结果如图：

import datetime
import random

import numpy as np


# 日期相减
def date_subtract(val1, val2):
    d1 = datetime.datetime.strptime(val1, '%Y-%m-%d %H:%M:%S.%f')
    d2 = datetime.datetime.strptime(val2, '%Y-%m-%d %H:%M:%S.%f')
    # print(d1, d2, (d1 - d2))
    return str(d1 - d2)[:-3]


dict1 = {}
dict2 = {}

for i in np.linspace(2, 6, num=5):
    day = int(i) + 2
    hour = int(i) + 3
    minute = int(i) + 10
    second = int(i) + 30
    millsecond = random.randint(100,999)
    dict2[int(i)] = '2021-08-0' + str(day) + ' 0' + str(hour) + ':' + str(minute) + ":" + str(second) + '.' +str(millsecond)

for i in range(1, 9):
    millsecond = random.randint(100,999)
    dict1[i] = '2021-08-0' + str(i) + ' 0' + str(i) + ':0' + str(i) + ":0" + str(i) + '.' + str(millsecond)

print(dict1)
print(dict2)

for key1 in dict1.keys():
    if key1 in dict2.keys():
        dict1[key1] = date_subtract(dict1[key1], dict2[key1])

print('dict1-dict2差值：如下')
print(dict1)

这个时间格式怎么能减2021-06-24 07:30:36.0，减了有什么意义？