df = pd.DataFrame({'Name': ['A'], 'start': [datetime(2000, 1, 10)], 'end':
    [datetime(1970, 4, 29)]})
df['diff'] = map(lambda td: datetime(1, 1, 1) + td, list(df['start'] - df[
    'end']))
df['diff'] = df['diff'].apply(lambda d: '{0}y{1}m'.format(d.year - 1, d.
    month - 1))
calculate datetime-difference in years, months, etc. in a new pandas dataframe column