d = {"year": [2018, 2019, 2020],"day_of_year": [200, 365, 60]}
df = pd.DataFrame(d)
df
df["int_number"] = df["year"]*1000 + df["day_of_year"]
df
df["date"] = pd.to_datetime(df["int_number"], format = "%Y%j")#注意 "%Y%j" 中转化格式 j，正是对应 dayofyear 参数
df