title: pandas.to_datetime ValueError: cannot assemble the datetimes
body: <p>I'm trying to create a pandas datetime series using 3 pandas columns. When I call the to_datetime() method, it throws an error, even though I'm feeding it series inputs of int64 dtypes in proper year, month, day format.</p>
  
  <pre><code>pd.to_datetime(df[['year', 'month', 'day']])
  </code></pre>
  
  <p>The last line of the error stack is the weirdest, because it seems to show my values are of %Y%m%d format, yet it won't accept them.</p>
  
  <pre><code>---------------------------------------------------------------------------
  
  TypeError                                 Traceback (most recent call last)
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in _convert_listlike(arg, box, format, name, tz)
  
      302             try:
  
  --&gt; 303                 values, tz = tslib.datetime_to_datetime64(arg)
  
      304                 return DatetimeIndex._simple_new(values, name=name, tz=tz)
  
  
  
  pandas/_libs/tslib.pyx in pandas._libs.tslib.datetime_to_datetime64()
  
  
  
  TypeError: Unrecognized value type: &lt;class 'int'&gt;
  
  
  
  During handling of the above exception, another exception occurred:
  
  
  
  ValueError                                Traceback (most recent call last)
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in _assemble_from_unit_mappings(arg, errors)
  
      476     try:
  
  --&gt; 477         values = to_datetime(values, format='%Y%m%d', errors=errors)
  
      478     except (TypeError, ValueError) as e:
  
  
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in to_datetime(arg, errors, dayfirst, yearfirst, utc, box, format, exact, unit, infer_datetime_format, origin)
  
      372         from pandas import Series
  
  --&gt; 373         values = _convert_listlike(arg._values, True, format)
  
      374         result = Series(values, index=arg.index, name=arg.name)
  
  
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in _convert_listlike(arg, box, format, name, tz)
  
      305             except (ValueError, TypeError):
  
  --&gt; 306                 raise e
  
      307
  
  
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in _convert_listlike(arg, box, format, name, tz)
  
      272                         result = array_strptime(arg, format, exact=exact,
  
  --&gt; 273                                                 errors=errors)
  
      274                     except tslib.OutOfBoundsDatetime:
  
  
  
  pandas/_libs/tslibs/strptime.pyx in pandas._libs.tslibs.strptime.array_strptime()
  
  
  
  ValueError: time data 20170200 does not match format '%Y%m%d' (match)
  
  
  
  During handling of the above exception, another exception occurred:
  
  
  
  ValueError                                Traceback (most recent call last)
  
  &lt;ipython-input-118-cd147256a618&gt; in &lt;module&gt;()
  
  
  ----&gt; 2 pd.to_datetime(df[['year', 'month', 'day']])
  
  
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in to_datetime(arg, errors, dayfirst, yearfirst, utc, box, format, exact, unit, infer_datetime_format, origin)
  
      374         result = Series(values, index=arg.index, name=arg.name)
  
      375     elif isinstance(arg, (ABCDataFrame, MutableMapping)):
  
  --&gt; 376         result = _assemble_from_unit_mappings(arg, errors=errors)
  
      377     elif isinstance(arg, ABCIndexClass):
  
      378         result = _convert_listlike(arg, box, format, name=arg.name)
  
  
  
  /opt/python/python35/lib/python3.5/site-packages/pandas/core/tools/datetimes.py in _assemble_from_unit_mappings(arg, errors)
  
      478     except (TypeError, ValueError) as e:
  
      479         raise ValueError("cannot assemble the "
  
  --&gt; 480                          "datetimes: {error}".format(error=e))
  
      481
  
      482     for u in ['h', 'm', 's', 'ms', 'us', 'ns']:
  
  
  
  ValueError: cannot assemble the datetimes: time data 20170200 does not match format '%Y%m%d' (match)
  </code></pre>