BUG: indexing with monotonic decreasing DTI
Hi there, I'm trying to add a column of time using pandas. But some dates doesn't work fine in string format. I'm working on jupyter notebook with Anaconda 4.3.0

Here's what I'm trying to do:

```python
data1 = data.head()
date_list = ["2018-01-02", "2017-02-10","2016-03-10", "2015-03-15", "2014-03-16"]
date_index = pd.to_datetime(date_list)
data1['date'] = date_index
data1
```
And the output is shown below.
![image](https://user-images.githubusercontent.com/16774189/35281474-f66c7f82-008d-11e8-88cd-743bd4e106fc.png)

Then I try to set 'date' as the index of this data frame.
```python
data1 = data1.set_index("date")
print(data1.index)
data1
```
This looks reasonable, too.
![image](https://user-images.githubusercontent.com/16774189/35281438-d85e7f4a-008d-11e8-9c43-d2d00b71d3e0.png)

Something weird happens when I try to index data using:
```python
data1.loc['2018-01-02']
```
It raises KeyError!!! 
![image](https://user-images.githubusercontent.com/16774189/35281042-e5508b36-008c-11e8-935b-f8aec3e5e4c3.png)

But other dates work fine. For example:
```python
data1.loc['2015-03-15']
```
![image](https://user-images.githubusercontent.com/16774189/35281177-44411cdc-008d-11e8-8bbb-1fe5d959b320.png)

Why is that? 

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.13.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.2.2
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: 0.9999999
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: None
</details>
