将单独的“年”和“日”列转换为一个带有lubridate属性的“日期”列?

I have one column for 'year' and one column for 'day' (julian day) and I want to use these columns to make a 'date' (2002-12-03 or any format is okay). I've found a lot of I have one column for 'year' and one column for