Fix Xl(sx|wt)Writer always using date_format even if a datetime is supplied.

[filmor]

In the XlsxWriter and XlwtWriter implementations there is a minor bug resulting in the datetime_format being discarded (i.e. https://github.com/pydata/pandas/blob/master/pandas/io/excel.py#L662):

if isinstance(cell.val, datetime.datetime):
    num_format_str = self.datetime_format
if isinstance(cell.val, datetime.date):
    num_format_str = self.date_format

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

[jreback]

can you add a test for this?

pls add a release note as well (reference this PR) as this doesn't have an associated issue?

[filmor]

Adding a test is difficult, as this only shows in Excel itself (by showing 2014-05-08 instead of 2014-05-08 15:23 in a timestamp column). In particular rereading the file will give proper datetime or Timestamp objects again.

I'll add a release-note.

[jreback]

ok then

[jreback]

ping when pushed

[filmor]

Ping.

[jreback]

thanks!