python datetime到np.datetime64

Question

是否有理由在不指定单位datetime.datetime的情况下从us数组转换，将日期时间转换为全天？或者这是一个错误？

import numpy as np
import datetime
print(np.array([datetime.datetime(2018, 5, 15, 6, 0),datetime.datetime(2018, 5, 15, 6, 30)]).astype('datetime64[us]'))
print(np.array([datetime.datetime(2018, 5, 15, 6, 0),datetime.datetime(2018, 5, 15, 6, 30)]).astype('datetime64'))

输出

['2018-05-15T06:00:00.000000' '2018-05-15T06:30:00.000000']
['2018-05-15' '2018-05-15']

numpy版本1.14.3和python 3.6.3

从numpy文档中，我看不到datetime64的默认单位是day。来自https://docs.scipy.org/doc/numpy-1.14.0/reference/arrays.datetime.html

>>> np.array(['2001-01-01T12:00', '2002-02-03T13:56:03.172'], dtype='datetime64')
array(['2001-01-01T12:00:00.000-0600', '2002-02-03T13:56:03.172-0600'], dtype='datetime64[ms]')