怎么用一个python脚本将一个utc时间如1503936000，需要根据系统时间取前一天00:00:00转换为utc时间，求详细代码
In [1]: import datetime
In [2]: a = datetime.datetime.fromtimestamp(1503936000)
In [3]: a
Out[3]: datetime.datetime(2017, 8, 29, 0, 0)
In [5]: yesterday = a - datetime.timedelta(days=1)
In [6]: yesterday
Out[6]: datetime.datetime(2017, 8, 28, 0, 0)
In [7]: import time
In [8]: b = time.mktime(yesterday.timetuple())*1e3 + yesterday.microsecond/1e3
In [9]: b
Out[9]: 1503849600000.0