Aggregate with `pd.Series.nunique` in datetime column has weird result
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](http://stackoverflow.com/questions/41597179/pandas-counting-unique-datetime-values-in-group-by-gives-weird-values).


    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
