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How do I convert day-of-year to date? For example, what's the date for 104th day of 2004?

This is the opposite of How do you convert POSIX date to day of year in R?

M--
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Tomas
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4 Answers4

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I found that @TMS answer ignores the year specified in the origin, replacing it with the actual year you are executing the command (as @Shekeine found). Doing this seems to work fine though:

as.Date(104, origin = "2014-01-01")

CAUTION: day-of-year is zero based in this case!

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user2739472
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  • Thank you for this. I am not sure why my code no longer works (I think it was working back then). Added an important warning to your answer as well. – Tomas Sep 22 '16 at 12:50
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    As noted above, that code gives a zero based day of year (ie, Jan 1st is DOY=0). If you want the more typical 1-based day of year (ie, Jan 1st is DOY=1) you'd use `origin="2013-12-31"` – DirtStats Nov 20 '18 at 20:25
7

This is the way I do it: 

as.Date(104, format = "%j", origin = "1.1.2014")
# "2014-04-15"

PS: for those who wonder if answering my own question is encouraged, please look here: https://meta.stackexchange.com/questions/17463/can-i-answer-my-own-questions-even-if-i-knew-the-answer-before-asking

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Tomas
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  • Hi, your code above returns `[1] "2015-04-15"` for me. Running `as.Date(1, format = "%j", origin=paste0("1.1.", "2015"))` returns `"2015-01-02"`. What am I missing here? – shekeine May 09 '15 at 17:27
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    I am not sure why my code no longer works (I think it was working back then). I accepted the other answer. – Tomas Sep 22 '16 at 12:51
  • @shekeine I didn't understand what was wrong with your output. If you're concerned that it returns 2nd of January - I think the counting starts at 0 in this case. – vladli Feb 20 '19 at 15:33
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as.Date(2, origin = '2014-01-01')

As for the issue with above code giving the output as 2014-01-03.

You can just put the origin as the last day of the previous year.

as.Date(2, origin = '2013-12-31')

It works that way.

jizhihaoSAMA
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aaloksk
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0

For anyone working with many years of DOY data, here's an easy way to extract the date, month, and day from DOY using dplyr.

library(dplyr)
#
# create example dataframe 
x <- data.frame(doy=c(rep(1:5, 3)), 
                year=c(rep(2010, 5), rep(2011, 5), rep(2012, 5)))
#
# change doy to dates using mutate()
x %>% mutate(date_= as.Date(doy-1, origin=paste0(year, "-01-01")), 
              month= strftime(date_, "%m"), 
              day=strftime(date_,"%d"))  
# subtract 1 from doy because R uses a 0 base index 
# otherwise DOY=1 will be Jan. 2.
# output:
    doy year      date_ month day
1    1 2010 2010-01-01    01  01
2    2 2010 2010-01-02    01  02
3    3 2010 2010-01-03    01  03
4    4 2010 2010-01-04    01  04
5    5 2010 2010-01-05    01  05
6    1 2011 2011-01-01    01  01
7    2 2011 2011-01-02    01  02
8    3 2011 2011-01-03    01  03
9    4 2011 2011-01-04    01  04
10   5 2011 2011-01-05    01  05
11   1 2012 2012-01-01    01  01
12   2 2012 2012-01-02    01  02
13   3 2012 2012-01-03    01  03
14   4 2012 2012-01-04    01  04
15   5 2012 2012-01-05    01  05
BonnieM
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