How floor a date to the first date of that month?

Question

I have a pandas DataFrame with index column = date.

Input:

            value
date    
1986-01-31  22.93
1986-02-28  15.46

Answer 1

Liked Mikhail Venkov answer. Added below code to have the column added as a timestamp value and keep timezone information

    df['month'] = pd.to_datetime(df['timestamp'].dt.strftime('%Y-%m-01')).dt.tz_localize(timezone) 

where timezone = 'America/Los_Angeles' or whatever zone you want