df2 = pd.merge(df, df1, on='Datetime', how='outer')Adding ‘how=’outer’ to ‘pd.merge()’ will actually do the job!
Here’s a very good comprehensive article about Panda’s Join, Merge and Concatenate.
And by the way, if you would like to fill NA values when merging, just add ‘.fillna(‘0′)’, in this case it will fill NA values with zeroes.