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

Here's another 'pandonic' way to do it:

df.date - pd.Timedelta('1 day') * (df.date.dt.day - 1)