I am working with a data set that looks a bit like this:
Year Date Day_nr Value 1976 19-02-1976 50 167 1976 19-03-1976 79 140 1
Google appears to have an answer: https://stat.ethz.ch/pipermail/r-help/2012-March/308013.html
Using first line from second set above as an example:
> strptime(paste("1976", 53), format="%Y %j") [1] "1976-02-22"