BUG: Fix Xl(sx|wt)Writer always using date_format.

Since `datetime.datetime` derives from `datetime.date` the second
`if`-clause will always trigger, leading to the `num_format_str` being
overwritten by the `date_format`.

Author: filmor

doc/source/release.rst

@@ -478,6 +478,7 @@ Bug Fixes
 - Bug in ``Float64Index.isin()`` where containing ``nan`` s would make indices
   claim that they contained all the things (:issue:`7066`).
 - Bug in ``DataFrame.boxplot`` where it failed to use the axis passed as the ``ax`` argument (:issue:`3578`)
+- Bug in the ``XlsxWriter`` and ``XlwtWriter`` implementations that resulted in datetime columns being formatted without the time (:issue:`7075`)
 
 pandas 0.13.1
 -------------

pandas/io/excel.py

@@ -661,7 +661,7 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0):
             num_format_str = None
             if isinstance(cell.val, datetime.datetime):
                 num_format_str = self.datetime_format
-            if isinstance(cell.val, datetime.date):
+            elif isinstance(cell.val, datetime.date):
                 num_format_str = self.date_format
 
             stylekey = json.dumps(cell.style)
@@ -782,7 +782,7 @@ def write_cells(self, cells, sheet_name=None, startrow=0, startcol=0):
             num_format_str = None
             if isinstance(cell.val, datetime.datetime):
                 num_format_str = self.datetime_format
-            if isinstance(cell.val, datetime.date):
+            elif isinstance(cell.val, datetime.date):
                 num_format_str = self.date_format
 
             stylekey = json.dumps(cell.style)