How do you convert POSIX date to day of year?

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An alternative is to format the "POSIXt" object using strftime():

R> today <- Sys.time()
R> today
[1] "2012-10-19 19:12:04 BST"
R> doy <- strftime(today, format = "%j")
R> doy
[1] "293"
R> as.numeric(doy)
[1] 293

which is preferable to remembering that the day of the years is zero-based in the POSIX standard.

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