Weird datetime string default parsing on to_datetime 
When using default `to_datetime`, without format, the answers is sometimes a bit weird:

```python
>>> import pandas as pd
>>> pd.to_datetime("2017-07-04 16-51-10")
Timestamp('2017-07-05 02:00:00')
```

Year and month are guessed correctly, day gets shifted by 1, and hour, minutes, seconds are completely off.

(edit) - Obviously the function works fine when passing the datetime format, it is the default guess that is weird.