DateTimeIndex appending with loc inconsistency in handling numpy datetime64
This is in Pandas `0.15.2` (but I also tried it on '0.15.2-148-g484f668'):

``` python
df = pd.DataFrame()
df.loc[np.datetime64(datetime.datetime.now()),'one'] = 100
df.loc[np.datetime64(datetime.datetime.now()),'one'] = 100
```

This is the output:

``` text
                               one
2015-02-18 14:50:05.606510     100
1970-01-17 11:37:51.007748755  100
```

It seems the the first usage of `loc` correctly works around limitations of `datetime64` but I cannot explain the second one.
