Assign back converted multiple columns to datetime failed
I want convert multiple columns to datetimes:

    df = pd.DataFrame(np.random.choice(['2015-01-01','2016-01-01'], size=(5,6))).add_prefix('date')
    print (df)
            date0       date1       date2       date3       date4       date5
    0  2015-01-01  2016-01-01  2015-01-01  2015-01-01  2016-01-01  2016-01-01
    1  2016-01-01  2016-01-01  2016-01-01  2015-01-01  2016-01-01  2015-01-01
    2  2015-01-01  2015-01-01  2016-01-01  2016-01-01  2016-01-01  2015-01-01
    3  2016-01-01  2015-01-01  2015-01-01  2015-01-01  2015-01-01  2015-01-01
    4  2015-01-01  2016-01-01  2015-01-01  2016-01-01  2016-01-01  2015-01-01
    
    i= range(0,6,2)
    df.iloc[:,i] = df.iloc[:,i].apply(lambda x: pd.to_datetime(x, errors='coerce'))
    print (df.dtypes)
    date0    object
    date1    object
    date2    object
    date3    object
    date4    object
    date5    object
    dtype: object
    
    print (df)
                     date0       date1                date2       date3  \
    0  1420070400000000000  2016-01-01  1420070400000000000  2015-01-01   
    1  1451606400000000000  2016-01-01  1451606400000000000  2015-01-01   
    2  1420070400000000000  2015-01-01  1451606400000000000  2016-01-01   
    3  1451606400000000000  2015-01-01  1420070400000000000  2015-01-01   
    4  1420070400000000000  2016-01-01  1420070400000000000  2016-01-01   
    
                     date4       date5  
    0  1451606400000000000  2016-01-01  
    1  1451606400000000000  2015-01-01  
    2  1451606400000000000  2015-01-01  
    3  1420070400000000000  2015-01-01  
    4  1451606400000000000  2015-01-01  

I test converting:

    print (df.iloc[:,i].apply(lambda x: pd.to_datetime(x, errors='coerce')).dtypes)
    date0    datetime64[ns]
    date2    datetime64[ns]
    date4    datetime64[ns]
    dtype: object

But assign back convert datetimes to unix dates. Also I test `loc` and same problem.

    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 60 Stepping 3, GenuineIntel
    byteorder: little
    LC_ALL: None
    LANG: sk_SK
    LOCALE: None.None
    
    pandas: 0.22.0
    pytest: 2.8.5
    pip: 9.0.1
    setuptools: 38.4.0
    Cython: 0.23.4
    numpy: 1.12.1
    scipy: 0.19.0
    pyarrow: 0.7.0
    xarray: None
    IPython: 4.1.2
    sphinx: 1.3.1
    patsy: 0.4.1
    dateutil: 2.5.1
    pytz: 2016.2
    blosc: None
    bottleneck: 1.2.1
    tables: 3.2.2
    numexpr: 2.6.2
    feather: 0.4.0
    matplotlib: 2.0.0
    openpyxl: 2.4.8
    xlrd: 0.9.4
    xlwt: 1.0.0
    xlsxwriter: 0.8.4
    lxml: 3.6.0
    bs4: 4.6.0
    html5lib: 0.999
    sqlalchemy: 1.0.12
    pymysql: None
    psycopg2: None
    jinja2: 2.8
    s3fs: None
    fastparquet: None
    pandas_gbq: None
    pandas_datareader: 0.4.0
    None
