Python - 根据一年中的时间将小时从EDT / EST转换为UTC

Question

东部标准时间是-5（到UTC）

东部夏令时为-4（至UTC）

那么为什么这不能正常工作？

>>> import datetime, pytz

# 5 hour difference during Eastern Standard Time
>>> datetime.datetime(2018, 1, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).hour
>>> 16

>>> datetime.datetime(2018, 1, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).astimezone(pytz.utc).hour
>>> 21

# 4 hour difference during Eastern Daylight Time
>>> datetime.datetime(2018, 4, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).hour
>>> 16

# So why does this not say 20?
>>> datetime.datetime(2018, 4, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).astimezone(pytz.utc).hour
>>> 21