python：日期计算

python语言中的datetime模块可以利用其中的方法获取不同的日期，比如获取当前日期、明天、昨天、上个月、下个月和明年。下面利用几个实例说明这些日期的获取方法，操作如下：

第一步，利用datetime模块获取当前日期
datetime.date.today();
如下图所示：

第二步，获取当前日期前一天日期，利用当前日期减去一天，如下图所示：

第三步，获取当前日期后一天日期，利用当前日期加上一天，如下图所示：

第四步，获取当前日期下一个月日期，利用当前日期加上30天，如下图所示：

第五步，获取当前日期上一个月的日期，利用当前日期减去30天，如下图所示：

第六步，获取当前日期返回明年今天的日期，利用当前日期加上365天，如下图所示：

# -*- coding: utf-8 -*-

#-----------------------------------------------------------------------------------
import datetime
#获取366天前的日期
day=(datetime.date.today() - datetime.timedelta(days=366)).strftime('%Y-%m-%d')
print(day)
#获取366天后的日期
day=(datetime.date.today() + datetime.timedelta(days=366)).strftime('%Y-%m-%d')
print(day)
#3周前期
day=(datetime.date.today() + datetime.timedelta(weeks=-3)).strftime('%Y-%m-%d')
print(day)
#-----------------------------------------------------------------------------------

'''获取当前日期前后N天或N月的日期'''

from time import strftime, localtime
from datetime import timedelta, date
import calendar

year = strftime("%Y", localtime())
mon = strftime("%m", localtime())
day = strftime("%d", localtime())
hour = strftime("%H", localtime())
min = strftime("%M", localtime())
sec = strftime("%S", localtime())

def today():
    '''''
    get today,date format="YYYY-MM-DD"
    '''''
    return date.today()

def todaystr():
    '''
    get date string, date format="YYYYMMDD"
    '''
    return year + mon + day

def datetime():
    '''''
    get datetime,format="YYYY-MM-DD HH:MM:SS"
    '''
    return strftime("%Y-%m-%d %H:%M:%S", localtime())

def datetimestr():
    '''''
    get datetime string
    date format="YYYYMMDDHHMMSS"
    '''
    return year + mon + day + hour + min + sec

def get_day_of_day(n=0):
    '''''
    if n>=0,date is larger than today
    if n<0,date is less than today
    date format = "YYYY-MM-DD"
    '''
    if (n < 0):
        n = abs(n)
        return date.today() - timedelta(days=n)
    else:
        return date.today() + timedelta(days=n)

def get_days_of_month(year, mon):
    '''''
    get days of month
    '''
    return calendar.monthrange(year, mon)[1]

def get_firstday_of_month(year, mon):
    '''''
    get the first day of month
    date format = "YYYY-MM-DD"
    '''
    days = ""
    if (int(mon) < 10):
        mon = "" + str(int(mon))
    arr = (year, mon, days)
    return "-".join("%s" % i for i in arr)

def get_lastday_of_month(year, mon):
    '''''
    get the last day of month
    date format = "YYYY-MM-DD"
    '''
    days = calendar.monthrange(year, mon)[1]
    mon = addzero(mon)
    arr = (year, mon, days)
    return "-".join("%s" % i for i in arr)

def get_firstday_month(n=0):
    '''''
    get the first day of month from today
    n is how many months
    '''
    (y, m, d) = getyearandmonth(n)
    d = ""
    arr = (y, m, d)
    return "-".join("%s" % i for i in arr)

def get_lastday_month(n=0):
    '''''
    get the last day of month from today
    n is how many months
    '''
    return "-".join("%s" % i for i in getyearandmonth(n))

def getyearandmonth(n=0):
    '''''
    get the year,month,days from today
    befor or after n months
    '''
    thisyear = int(year)
    thismon = int(mon)
    totalmon = thismon + n
    if (n >= 0):
        if (totalmon <= 12):
            days = str(get_days_of_month(thisyear, totalmon))
            totalmon = addzero(totalmon)
            return (year, totalmon, days)
        else:
            i = totalmon / 12
            j = totalmon % 12
            if (j == 0):
                i -= 1
                j = 12
            thisyear += i
            days = str(get_days_of_month(thisyear, j))
            j = addzero(j)
            return (str(thisyear), str(j), days)
    else:
        if ((totalmon > 0) and (totalmon < 12)):
            days = str(get_days_of_month(thisyear, totalmon))
            totalmon = addzero(totalmon)
            return (year, totalmon, days)
        else:
            i = totalmon / 12
            j = totalmon % 12
            if (j == 0):
                i -= 1
                j = 12
            thisyear += i
            days = str(get_days_of_month(thisyear, j))
            j = addzero(j)
            return (str(thisyear), str(j), days)

def addzero(n):
    '''''
    add 0 before 0-9
    return 01-09
    '''
    nabs = abs(int(n))
    if (nabs < 10):
        return "" + str(nabs)
    else:
        return nabs

def get_today_month(n=0):
    '''''
    获取当前日期前后N月的日期
    if n>0, 获取当前日期前N月的日期
    if n<0, 获取当前日期后N月的日期
    date format = "YYYY-MM-DD"
    '''
    (y, m, d) = getyearandmonth(n)
    arr = (y, m, d)
    if (int(day) < int(d)):
        arr = (y, m, day)
    return "-".join("%s" % i for i in arr)

if __name__ == "__main__":
    print today()#获取当前日期，2017-12-02
    print todaystr()#
    print datetime()#2017-12-02 16:37:19
    print datetimestr()#
    print get_day_of_day(20)#获取20天后的日期，2017-12-22
    print get_day_of_day(-3)#获取3天前的日期，2017-11-29
    print get_today_month(-3)#获取3个月前的日期，  2017-09-02
    print get_today_month(3)# 获取3个月后的日期， 2018-03-02
    print get_today_month(19)# 获取19个月后的日期，2019-07-02