Aggregate with pd.Series.nunique in datetime column has weird result

[jesrael]

I have DataFrame:

import pandas as pd
import datetime as datetime

a = pd.DataFrame({'id': [1, 2, 3, 2], 
              'my_date': [datetime.datetime(2017, 1, i) for i in range(1, 4)] + [datetime.datetime(2017, 1, 1)],
              'num': [2, 3, 1, 4]
        })

print (a.dtypes)
id                  int64
my_date    datetime64[ns]
num                 int64
dtype: object

I try aggreagate by pd.Series.nunique, but get wrong output for datetimes:

grouped_a = a.groupby('id').agg({'my_date': pd.Series.nunique, 
                                     'num': pd.Series.nunique}).reset_index()
grouped_a.columns = ['id', 'num_unique_num', 'num_unique_my_date']

print (grouped_a)
   id  num_unique_num            num_unique_my_date
0   1               1 1970-01-01 00:00:00.000000001
1   2               2 1970-01-01 00:00:00.000000002
2   3               1 1970-01-01 00:00:00.000000001

My solution is use nunique which works nice.

grouped_a = a.groupby('id').agg({'my_date': 'nunique', 
                                     'num': 'nunique'}).reset_index()
grouped_a.columns = ['id', 'num_unique_num', 'num_unique_my_date']
print (grouped_a)
   id  num_unique_num  num_unique_my_date
0   1               1                   1
1   2               2                   2
2   3               1                   1

But why does not work first solution? Bug?

SO question.

print (pd.show_versions())
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.1.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 42 Stepping 7, GenuineIntel
byteorder: little
LC_ALL: None
LANG: sk_SK
LOCALE: None.None

pandas: 0.19.2+0.g825876c.dirty
nose: 1.3.7
pip: 8.1.1
setuptools: 20.3
Cython: 0.23.4
numpy: 1.11.0
scipy: 0.17.0
statsmodels: 0.6.1
xarray: None
IPython: 4.1.2
sphinx: 1.3.1
patsy: 0.4.0
dateutil: 2.5.1
pytz: 2016.2
blosc: None
bottleneck: 1.0.0
tables: 3.2.2
numexpr: 2.5.1
matplotlib: 1.5.1
openpyxl: 2.3.2
xlrd: 0.9.4
xlwt: 1.0.0
xlsxwriter: 0.8.4
lxml: 3.6.0
bs4: 4.4.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: 1.0.12
pymysql: None
psycopg2: None
jinja2: 2.8
boto: 2.39.0
pandas_datareader: 0.2.1
None

[jreback]

duplicate of #14423

the fix actually is pretty simple, pls see the other issue.