Inconsistent behavior of partial string indexing vs straight indexing for timeseries data
I recently ran into an issue where pandas is not handling the "<=" logical operator when comparing dates in an interesting way.

```
import pandas as pd
rng = pd.date_range('1/1/2011', periods=72, freq='H')
df = pd.DataFrame(rng)
df.columns=['date']

crit = (df['date'] >= '2011-01-01') & (df['date'] <= '2011-01-02')
df.loc[crit, 'New_Col'] = 'First two days?'
```

Anything after 2011-01-02 00:00:00 is not included in the less than or equal to 2011-01-02. One would expect that less than or equal to mean that entire day. Is this by design, or is this a bug?
