You should be able to use join, which joins on the index as default. Given your desired result, you must use outer as the join type.
>>> df1.join(df2, how='outer')
V1 V2
A 1/1/2012 12 15
2/1/2012 14 NaN
3/1/2012 NaN 21
B 1/1/2012 15 24
2/1/2012 8 9
C 1/1/2012 17 NaN
2/1/2012 9 NaN
D 1/1/2012 NaN 7
2/1/2012 NaN 16
Signature: _.join(other, on=None, how='left', lsuffix='', rsuffix='', sort=False) Docstring: Join columns with other DataFrame either on index or on a key column. Efficiently Join multiple DataFrame objects by index at once by passing a list.