python 自己写的关于时间很有用的函数

from datetime import datetime
from datetime import timedelta
import calendar

#输入日期，以及频率默认1周（日、周、月或者年），输出往前日、周、月、年的日期
def get_pre_date(Date, fre = ‘1w‘):
    end_date = datetime.strptime(Date, ‘%Y%m%d‘)
    if fre.endswith(‘w‘):
        start_date = end_date - timedelta(int(fre[: -1]) * 7)
        start_date = start_date.strftime(‘%Y%m%d‘)
    elif fre.endswith(‘d‘):
        start_date = end_date - timedelta(int(fre[: -1]))
        start_date = start_date.strftime(‘%Y%m%d‘)
    elif fre.endswith(‘m‘):
        start_date_year = end_date.year - int(fre[: -1])// 12
        #余数
        remainder = int(fre[: -1])% 12
        delta_month = end_date.month - remainder        
        if delta_month <= 0:
            start_date_year -= 1
            start_date_month = 12 + remainder
        else:
            start_date_month = end_date.month - remainder
        #判断日子,通过月份来判断
        delta_day =calendar.monthrange(end_date.year, end_date.month)[1] - end_date.day
        start_date_day = calendar.monthrange(start_date_year, start_date_month)[1] - delta_day
        start_date = str(start_date_year) + str(start_date_month).zfill(2) + str(start_date_day)
    elif fre.endswith(‘y‘):
        start_date_year = end_date.year - int(fre[: -1])
        start_date = str(start_date_year) + str(end_date.month).zfill(2) + str(end_date.day)
    else:
        print(‘输入："nd", "nw", "nm", "ny"，其中"n"是数字\n‘)
    return start_date

get_pre_date(‘20150831‘, ‘16m‘)