Code Sample, a copy-pastable example if possible

In [2]: ts = 1.4884427244831782e+18

In [3]: pd.to_datetime(ts, utc=True)
Out[3]: Timestamp('2017-03-02 08:18:44.483178240+0000', tz='UTC')

In [4]: pd.to_datetime([ts], utc=True)
Out[4]: DatetimeIndex(['2017-03-02 08:18:44.483178240+00:00'], dtype='datetime64[ns, UTC]', freq=None)

In [5]: pd.to_datetime(pd.Series([ts]), utc=True)
Out[5]: 
0   2017-03-02 08:18:44.483178240
dtype: datetime64[ns]

Problem description

The argument utc=True in function to_datetime is not working when passing a Series, differently from other case, e.g. when passing a list or a scalar Unix timestamp.

Expected Output

The expected output is the same as the one given by

In [6]: pd.to_datetime(pd.Series([ts])).dt.tz_localize('UTC')
Out[6]: 
0   2017-03-02 08:18:44.483178240+00:00
dtype: datetime64[ns, UTC]

Another example: tz_convert

It is straightforward to convert to another time zone from a Unix timestamp or list:

In [7]: pd.to_datetime(ts, utc=True).tz_convert('Europe/Paris')
Out[7]: Timestamp('2017-03-02 09:18:44.483178240+0100', tz='Europe/Paris')

In [8]: pd.to_datetime([ts], utc=True).tz_convert('Europe/Paris')
Out[8]: DatetimeIndex(['2017-03-02 09:18:44.483178240+01:00'], dtype='datetime64[ns, Europe/Paris]', freq=None)

whereas for a Series:

In [9]: pd.to_datetime(pd.Series([ts])).dt.tz_localize('UTC').dt.tz_convert('Europe/Paris')
Out[9]: 
0   2017-03-02 09:18:44.483178240+01:00
dtype: datetime64[ns, Europe/Paris]

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.5.2.final.0 python-bits: 64 OS: Linux OS-release: 3.13.0-113-generic machine: x86_64 processor: x86_64 byteorder: little LC_ALL: None LANG: ca_ES.UTF-8 LOCALE: ca_ES.UTF-8 pandas: 0.19.2 nose: 1.3.7 pip: 8.1.2 setuptools: 27.2.0.post20161106 Cython: 0.24.1 numpy: 1.11.1 scipy: 0.18.1 statsmodels: 0.6.1 xarray: None IPython: 5.1.0 sphinx: 1.4.6 patsy: 0.4.1 dateutil: 2.5.3 pytz: 2016.6.1 blosc: None bottleneck: 1.1.0 tables: 3.2.3.1 numexpr: 2.6.1 matplotlib: 1.5.3 openpyxl: 2.3.2 xlrd: 1.0.0 xlwt: 1.1.2 xlsxwriter: 0.9.3 lxml: 3.6.4 bs4: 4.5.1 html5lib: None httplib2: None apiclient: None sqlalchemy: 1.0.13 pymysql: None psycopg2: None jinja2: 2.8 boto: 2.42.0 pandas_datareader: 0.2.1

Comment From: jorisvandenbossche

@albertvillanova Thanks for the report! This is a duplicate of #6415 PRs welcome!