print('pandas-version: ', pd.__version__)
df1 = pd.DataFrame.from_records([('2015-07-09 12:00:00', 1, 1, 1), (
    '2015-07-09 13:00:00', 1, 1, 1), ('2015-07-09 14:00:00', 1, 1, 1), (
    '2015-07-09 15:00:00', 1, 1, 1)], columns=['Dt', 'A', 'B', 'C']).set_index(
    'Dt')
df2 = pd.DataFrame.from_records([('2015-07-09 14:00:00', 2, 2, 2, 2), (
    '2015-07-09 15:00:00', 2, 2, 2, 2), ('2015-07-09 16:00:00', 2, 2, 2, 2),
    ('2015-07-09 17:00:00', 2, 2, 2, 2)], columns=['Dt', 'A', 'B', 'C', 'D']
    ).set_index('Dt')
res_combine1st = df2.combine_first(df1)
Setting DataFrame values with enlargement