iterate over index and define each range as a day

[perfringo]

I don't know whether it addresses your problem:

import pandas as pd

data = ['2019-05-02 00:03:00', '2019-05-02 00:08:00', '2019-05-02 00:13:00', '2019-05-02 00:18:00',
        '2019-05-02 00:23:00', '2019-05-02 00:28:00', '2019-05-02 00:33:00']

df = pd.DataFrame({'num': range(len(data))}, index=pd.to_datetime(data))

It will create following DataFrame with DatetimeIndex:

Output:
                    num
2019-05-02 00:03:00    0
2019-05-02 00:08:00    1
2019-05-02 00:13:00    2
2019-05-02 00:18:00    3
2019-05-02 00:23:00    4
2019-05-02 00:28:00    5
2019-05-02 00:33:00    6

Now we can set num value based on time:

df.iloc[df.index.indexer_between_time('00:13:00', '23:59:00')] = 20

Which will give:

Output:
                    num
2019-05-02 00:03:00    0
2019-05-02 00:08:00    1
2019-05-02 00:13:00   20
2019-05-02 00:18:00   20
2019-05-02 00:23:00   20
2019-05-02 00:28:00   20
2019-05-02 00:33:00   20