<h4 dir="auto">Code Sample, a copy-pastable example if possible</h4>
<div class="highlight highlight-source-python notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="# First, create a `Series` with `dtype` int64 or float64 or object (any type but datetime64).
series = pd.Series({'value': 123})
# &gt;&gt;&gt; series
# value    123
# dtype: int64

# Setting a new datetime64 value, and it will be converted to int64.
series.at['date'] = datetime.datetime.now()
# &gt;&gt;&gt; series
# value                    123
# date     1520605566114680000
# dtype: object

# If assigning to a existed field, it won't be auto-converted.
series.at['date'] = datetime.datetime.now()
# &gt;&gt;&gt; series
# value                           123
# date     2018-03-09 14:26:19.945935
# dtype: object"><pre class="notranslate"><span class="pl-c"># First, create a `Series` with `dtype` int64 or float64 or object (any type but datetime64).</span>
<span class="pl-s1">series</span> <span class="pl-c1">=</span> <span class="pl-s1">pd</span>.<span class="pl-v">Series</span>({<span class="pl-s">'value'</span>: <span class="pl-c1">123</span>})
<span class="pl-c"># &gt;&gt;&gt; series</span>
<span class="pl-c"># value    123</span>
<span class="pl-c"># dtype: int64</span>

<span class="pl-c"># Setting a new datetime64 value, and it will be converted to int64.</span>
<span class="pl-s1">series</span>.<span class="pl-s1">at</span>[<span class="pl-s">'date'</span>] <span class="pl-c1">=</span> <span class="pl-s1">datetime</span>.<span class="pl-s1">datetime</span>.<span class="pl-en">now</span>()
<span class="pl-c"># &gt;&gt;&gt; series</span>
<span class="pl-c"># value                    123</span>
<span class="pl-c"># date     1520605566114680000</span>
<span class="pl-c"># dtype: object</span>

<span class="pl-c"># If assigning to a existed field, it won't be auto-converted.</span>
<span class="pl-s1">series</span>.<span class="pl-s1">at</span>[<span class="pl-s">'date'</span>] <span class="pl-c1">=</span> <span class="pl-s1">datetime</span>.<span class="pl-s1">datetime</span>.<span class="pl-en">now</span>()
<span class="pl-c"># &gt;&gt;&gt; series</span>
<span class="pl-c"># value                           123</span>
<span class="pl-c"># date     2018-03-09 14:26:19.945935</span>
<span class="pl-c"># dtype: object</span></pre></div>
<h4 dir="auto">Problem description</h4>
<p dir="auto">When assigning a <code class="notranslate">datetime64</code> value to a new field of a non-<code class="notranslate">datetime64</code> <code class="notranslate">Series</code>, it will be converted to <code class="notranslate">int64</code>.<br>
While assigning to a existed field, it won't be converted.</p>
<h4 dir="auto">Expected Output</h4>
<p dir="auto">A <code class="notranslate">datetime64</code> value should never be implicitly converted to <code class="notranslate">int64</code>.</p>
<h4 dir="auto">Output of <code class="notranslate">pd.show_versions()</code></h4>
<details>
<h2 dir="auto">INSTALLED VERSIONS</h2>
<p dir="auto">commit: None<br>
python: 3.6.4.final.0<br>
python-bits: 64<br>
OS: Darwin<br>
OS-release: 17.4.0<br>
machine: x86_64<br>
processor: i386<br>
byteorder: little<br>
LC_ALL: None<br>
LANG: zh_CN.UTF-8<br>
LOCALE: zh_CN.UTF-8</p>
<p dir="auto">pandas: 0.22.0<br>
pytest: None<br>
pip: 9.0.1<br>
setuptools: 38.5.2<br>
Cython: None<br>
numpy: 1.14.1<br>
scipy: 1.0.0<br>
pyarrow: None<br>
xarray: None<br>
IPython: None<br>
sphinx: None<br>
patsy: None<br>
dateutil: 2.6.1<br>
pytz: 2018.3<br>
blosc: None<br>
bottleneck: None<br>
tables: None<br>
numexpr: None<br>
feather: None<br>
matplotlib: 2.2.0<br>
openpyxl: None<br>
xlrd: 1.1.0<br>
xlwt: None<br>
xlsxwriter: 1.0.2<br>
lxml: None<br>
bs4: 4.6.0<br>
html5lib: None<br>
sqlalchemy: None<br>
pymysql: 0.7.11.None<br>
psycopg2: None<br>
jinja2: None<br>
s3fs: None<br>
fastparquet: None<br>
pandas_gbq: None<br>
pandas_datareader: None</p>
</details>
<p dir="auto">xref <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="175624003" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/14179" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/14179/hovercard" href="https://github.com/pandas-dev/pandas/issues/14179">#14179</a><br>
xref <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="137459758" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/12499" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/12499/hovercard" href="https://github.com/pandas-dev/pandas/issues/12499">#12499</a><br>
xref <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="269507526" data-permission-text="Title is private" data-url="https://github.com/pandas-dev/pandas/issues/18027" data-hovercard-type="issue" data-hovercard-url="/pandas-dev/pandas/issues/18027/hovercard" href="https://github.com/pandas-dev/pandas/issues/18027">#18027</a></p>
<p dir="auto"><a href="http://stackoverflow.com/questions/23250333/in-pandas-dataframe-after-i-set-an-entire-column-i-cant-update-another-column" rel="nofollow">http://stackoverflow.com/questions/23250333/in-pandas-dataframe-after-i-set-an-entire-column-i-cant-update-another-column</a></p>
<p dir="auto">but <code class="notranslate">.loc</code> works</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="In [50]: df = pd.DataFrame(index=pd.date_range(start,periods=1), columns=['timenow','Live'])

In [51]: df.at[start,'timenow'] = datetime.today() # initial value

In [52]: df
Out[52]: 
                              timenow Live
2014-04-01 2014-04-23 12:42:38.883082  NaN

[1 rows x 2 columns]

In [53]: df._data
Out[53]: 
BlockManager
Items: Index([u'timenow', u'Live'], dtype='object')
Axis 1: &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
[2014-04-01]
Length: 1, Freq: D, Timezone: None
ObjectBlock: [timenow, Live], 2 x 1, dtype: object

In [54]: df = pd.DataFrame(index=pd.date_range(start,periods=1), columns=['timenow','Live'])

In [55]: df.loc[start,'timenow'] = datetime.today() # initial value

In [56]: df
Out[56]: 
                              timenow Live
2014-04-01 2014-04-23 12:43:00.554915  NaT

[1 rows x 2 columns]

In [57]: df._data
Out[57]: 
BlockManager
Items: Index([u'timenow', u'Live'], dtype='object')
Axis 1: &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
[2014-04-01]
Length: 1, Freq: D, Timezone: None
DatetimeBlock: [timenow, Live], 2 x 1, dtype: datetime64[ns]
"><pre class="notranslate"><code class="notranslate">In [50]: df = pd.DataFrame(index=pd.date_range(start,periods=1), columns=['timenow','Live'])

In [51]: df.at[start,'timenow'] = datetime.today() # initial value

In [52]: df
Out[52]: 
                              timenow Live
2014-04-01 2014-04-23 12:42:38.883082  NaN

[1 rows x 2 columns]

In [53]: df._data
Out[53]: 
BlockManager
Items: Index([u'timenow', u'Live'], dtype='object')
Axis 1: &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
[2014-04-01]
Length: 1, Freq: D, Timezone: None
ObjectBlock: [timenow, Live], 2 x 1, dtype: object

In [54]: df = pd.DataFrame(index=pd.date_range(start,periods=1), columns=['timenow','Live'])

In [55]: df.loc[start,'timenow'] = datetime.today() # initial value

In [56]: df
Out[56]: 
                              timenow Live
2014-04-01 2014-04-23 12:43:00.554915  NaT

[1 rows x 2 columns]

In [57]: df._data
Out[57]: 
BlockManager
Items: Index([u'timenow', u'Live'], dtype='object')
Axis 1: &lt;class 'pandas.tseries.index.DatetimeIndex'&gt;
[2014-04-01]
Length: 1, Freq: D, Timezone: None
DatetimeBlock: [timenow, Live], 2 x 1, dtype: datetime64[ns]

</code></pre></div>