python - Pandas: Change day

Question

I have a series in datetime format, and need to change the day to 1 for each entry. I have thought of numerous simple solutions, but none of them works for me. For now, the only thing that actually works is

• set the series as the index
• Query month and year from the index
• Reconstruct a new time series using year, month and 1

It can't really be that complicated, can it? There is month start, but is unfortunately an offset, that's of no use here. There seems to be no set() function for the method, and even less functionality while the series is a column, and not (part of) the index itself.

The only related question was this, but the trick used there is not applicable here.

Answer 1

You can use .apply and datetime.replace, eg:

import pandas as pd
from datetime import datetime

ps = pd.Series([datetime(2014, 1, 7), datetime(2014, 3, 13), datetime(2014, 6, 12)])
new = ps.apply(lambda dt: dt.replace(day=1))

Gives:

0   2014-01-01
1   2014-03-01
2   2014-06-01
dtype: datetime64[ns]