Google似乎有一个答案:

Google appears to have an answer: https://stat.ethz.ch/pipermail/r-help/2012-March/308013.html

以上面第二组中的第一行为例:

Using first line from second set above as an example:

> strptime(paste("1976", 53), format="%Y %j")
[1] "1976-02-22"