df = pd.DataFrame({'doy': [49, 65, 81, 97, 113, 129, 145, 161], 'year': [
    2000, 2000, 2000, 2001, 2001, 2001, 2001, 2001]})
df.index = compose_date(df['year'], days=df['doy'])
Converting year and day of year into datetime index in pandas