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我有一个Python datetime.datetime对象.减去一天的最佳方法是什么？

我试图从值中减去一个日期值datetime.today()来计算多久以前的东西.但它抱怨说:

TypeError: can't subtract offset-naive and offset-aware datetimes

该值datetime.today()似乎不是"时区感知",而我的其他日期值是.如何获得值datetime.today()是时区的？现在它给我时间在当地时间,恰好是PST,即UTC-8hrs.最坏的情况是,有没有办法我可以手动输入时区值到datetime返回的对象datetime.today()并将其设置为UTC-8？当然,理想的解决方案是让它自动知道时区.

我有以下方法:

# last_updated is a datetime() object, representing the last time this program ran
def time_diff(last_updated):
    day_period = last_updated.replace(day=last_updated.day + 1, 
                                      hour=1,
                                      minute=0,  
                                      second=0,
                                      microsecond=0)
    delta_time = day_period - last_updated
    hours = delta_time.seconds // 3600
    # make sure a period of 24hrs have passed before shuffling
    if hours >= 24:
        print "hello"
    else:
        print "do nothing"

我想知道从那以后24小时过去了last_updated,我怎么能这样做last_updated呢？