一、输入日期，判断这一天是这一年的第几天？

import datetime 
def dayofyear(): 
	year = input("请输入年份: ") 
	month = input("请输入月份: ") 
	day = input("请输入天: ") 
	date1 = datetime.date(year=int(year),month=int(month),day=int(day)) 
	date2 = datetime.date(year=int(year),month=1,day=1) 
	return (date1-date2).days+1

二、怎么实现时间的加减计算？

保证都是同一个类型才能加减，要么都是datetime，date

基于年，月，天，小时，分钟，秒，毫秒的加减计算？

需求：从现在开始，两天之后，两个月之后，一年之后分别对于的时间是？

1、timedelta
2、dateutil.relativedelta

from datetime import timedelta,datetime
from time import strftime
from dateutil.relativedelta import relativedelta

now_day=datetime.today()
print(now_day)

#todo 5分钟后的日期
min_day=now_day+timedelta(minutes=5)
print(min_day)

#todo 5小时后的日期
hour_5=now_day+timedelta(hours=5)
print(hour_5)

#todo 5周后的日期
week_5=now_day+timedelta(weeks=5)
print(week_5)

#todo 5天后的日期
day_5=now_day+timedelta(days=5)
print(day_5)

#todo 7个月后的日期
month_7=now_day+relativedelta(months=7)
print(month_7)

#todo 5年后的日期
year_5=now_day+relativedelta(years=5)
print(year_5)

执行结果：
2022-08-02 21:25:53.817425
2022-08-02 21:30:53.817425
2022-08-03 02:25:53.817425
2022-09-06 21:25:53.817425
2022-08-07 21:25:53.817425
2023-03-02 21:25:53.817425
2027-08-02 21:25:53.817425

特别注意：dateutil的安装方式为：pip3 install python-dateutil

pip3 install python-dateutil

三、计算上一个周五的日期

from datetime import datetime, timedelta

weekdays = ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday']


def get_previous_byday(dayname, start_date=None):
    if start_date is None:
        start_date = datetime.today()
    day_num = start_date.weekday()          #是属于一周的第几天，计数从0开始
    day_num_target = weekdays.index(dayname)    #上周的第几天
    days_ago = 7 + day_num - day_num_target     #相差的时间
    if days_ago == 0:
        days_ago = 7
    target_date = start_date - timedelta(days=days_ago)
    return target_date

print(get_previous_byday('Wednesday'))