Take care with series.astype('O') and DataFrame.value with datetime64
```
In [7]: s
Out[7]: 
0   2000-01-01 00:00:00
1   2000-01-02 00:00:00
2   2000-01-03 00:00:00
3   2000-01-04 00:00:00
4   2000-01-05 00:00:00
5   2000-01-06 00:00:00
6   2000-01-07 00:00:00
7   2000-01-08 00:00:00
8   2000-01-09 00:00:00
9   2000-01-10 00:00:00

In [8]: s.astype('O')
Out[8]: 
0    1970-01-11 184:00:00
1    1970-01-11 208:00:00
2    1970-01-11 232:00:00
3     1970-01-11 00:00:00
4     1970-01-11 24:00:00
5     1970-01-11 48:00:00
6     1970-01-11 72:00:00
7     1970-01-11 96:00:00
8    1970-01-11 120:00:00
9    1970-01-11 144:00:00
```
