How to set the index of a pandas DataFrame to match the index of a datetime series?

Question

I have a pandas DataFrame of a datetime series and a pandas DataFrame with a numbered index:

​How do I set the index of the DataFrame with the numbered index to match the index of the DataFrame with the datetime series?

Answer 1

If same length of both DataFrames :

df1.index = df2.index

Or:

df1= df1.set_index(df2.index)

Answer 2

df.index = pd.PeriodIndex(df.Date, freq='D')