Format date-time as seasons in R? - r

Question

In R, it's possible to format POSIXlt date-time objects as a month:
format(Sys.time(), format='%Y-%m')
Is there a way to do the same thing with seasons, or 3-month groups (DJF, MAM, JJA, SON)? These divisions are really common in climatological and ecological science, and it would be great to have a neat way to format them quickly like with months. Obviously DJF falls over 2 years, but for the purposes or this question, that doesn't really matter - just consistently shove them into either year, (or, ideally, it would be good to be able to specify which year they go into).
I'm using the output as a index for by(), so the output format doesn't matter much, just as long as each year/season is unique.
Edit: example data:
dates <- Sys.Date()+seq(1,380, by=35)
dates <- structure(c(16277, 16312, 16347, 16382, 16417, 16452, 16487,
16522, 16557, 16592, 16627), class = "Date")
dates
#[1] "2014-07-26" "2014-08-30" "2014-10-04" "2014-11-08" "2014-12-13"
# "2015-01-17" "2015-02-21" "2015-03-28" "2015-05-02" "2015-06-06" "2015-07-11"
should result in:
c("2014-JJA", "2014-JJA", "2014-SON", "2014-SON", "2015-DJF", "2015-DJF",
"2015-DJF", "2015-MAM", "2015-MAM", "2015-JJA", "2015-JJA")
But the "2015-DJF"s could also be "2014-DJF". Also, the form of the output doesn't matter - "2104q4 or 201404 would also be fine.

Answer 1

as.POSIXlt returns a named list (which makes it unsuitable for data.frame columns). The list columns can be individually accessed and include "year" (1900-based, unlike 1970 used for default) and "mon" (0-based). Best place to see this list in hte help system is ?DateTimeClasses:
First just a Seasons calculation, then a Year-Seasons calculation
c('DJF', 'MAM', 'JJA', 'SON')[ # select from character vector with numeric vector
1+((as.POSIXlt(dates)$mon+1) %/% 3)%%4]
[1] "JJA" "JJA" "SON" "SON" "DJF" "DJF" "DJF" "MAM" "MAM" "JJA"
[11] "JJA"
paste( 1900 + # this is the base year for POSIXlt year numbering
as.POSIXlt( dates )$year +
1*(as.POSIXlt( dates )$year==12) , # offset needed for December
c('DJF', 'MAM', 'JJA', 'SON')[ # indexing from 0-based-mon
1+((as.POSIXlt(dates)$mon+1) %/% 3)%%4]
, sep="-")
[1] "2014-JJA" "2014-JJA" "2014-SON" "2014-SON" "2014-DJF"
[6] "2015-DJF" "2015-DJF" "2015-MAM" "2015-MAM" "2015-JJA"
[11] "2015-JJA"
Shouldn't be that difficult to make a function that constructs the formatting you expect. This is just modulo arithmetic on the POSIXlt values for month and year.

Answer 2

I like using a lookup vector for these sorts of problems, e.g.:
x <- as.POSIXlt(
seq.Date(as.Date("2000-01-01"),as.Date("2002-01-01"),by="2 months")
)
E.g., if you want to specify southern hemisphere seasons, you could do:
src <- rep(c("su","au","wi","sp"),each=3)[c(2:12,1)]
paste(format(x,"%Y-%m"),src[x$mon+1])
# [1] "2000-01 su" "2000-03 au" "2000-05 au" "2000-07 wi" "2000-09 sp"
# [6] "2000-11 sp" "2001-01 su" "2001-03 au" "2001-05 au" "2001-07 wi"
#[11] "2001-09 sp" "2001-11 sp" "2002-01 su"
Change the src names as you see fit to relabel the categories.

Answer 3

Let Q1 be DJF; Q2, MAM; etc. then:
seasonal.quarters <- function(x) {
x <- as.POSIXlt(x)
x$mon <- (x$mon + 1) %% 12
quarters(x)
}
options(stringsAsFactors=FALSE)
nonleap.year <- seq(from=as.POSIXct('2013-1-1'), to=as.POSIXct('2014-1-1'), by='day')
d <- data.frame(ms=months(nonleap.year), qs=seasonal.quarters(nonleap.year))
by(d, INDICES=list(d$qs), FUN=function(x) unique(x$ms))
# : Q1
# [1] "January" "February" "December"
# -------------------------------------
# : Q2
# [1] "March" "April" "May"
# -------------------------------------
# : Q3
# [1] "June" "July" "August"
# -------------------------------------
# : Q4
# [1] "September" "October" "November"
leap.year <- seq(from=as.POSIXct('2016-1-1'), to=as.POSIXct('2017-1-1'), by='day')
d <- data.frame(ms=months(leap.year), qs=seasonal.quarters(leap.year))
by(d, INDICES=list(d$qs), FUN=function(x) unique(x$ms))
# : Q1
# [1] "January" "February" "December"
# -------------------------------------
# : Q2
# [1] "March" "April" "May"
# -------------------------------------
# : Q3
# [1] "June" "July" "August"
# -------------------------------------
# : Q4
# [1] "September" "October" "November"

Answer 4

This is an alternative to 42-'s answer above. Go see my comment there for the reason I posted it.
dates_orig <- as.POSIXlt(c("2013-01-01", "2013-02-01", "2013-03-01", "2013-04-01", "2013-05-01", "2013-06-01", "2013-07-01", "2013-08-01", "2013-09-01", "2013-10-01", "2013-11-01", "2013-12-01", "2014-01-01", "2014-02-01", "2014-03-01", "2014-04-01", "2014-05-01", "2014-06-01", "2014-07-01", "2014-08-01", "2014-09-01", "2014-10-01", "2014-11-01", "2014-12-01"))
format(dates_orig, format='%Y%b')
[1] "2013Jan" "2013Feb" "2013Mar" "2013Apr" "2013May" "2013Jun" "2013Jul" "2013Aug" "2013Sep" "2013Oct" "2013Nov" "2013Dec" "2014Jan" "2014Feb" "2014Mar"
[16] "2014Apr" "2014May" "2014Jun" "2014Jul" "2014Aug" "2014Sep" "2014Oct" "2014Nov" "2014Dec"
dates <- as.POSIXlt(dates_orig)
# shift Jan and Feb to the previous year
dates$year[dates$mon < 2] <- dates$year[dates$mon < 2] - 1
# convert months to seasons (named by first month of season)
dates$mon <- (((dates$mon - 2) %/% 3) %% 4) * 3 + 2
format(dates, format='%Y%b')
[1] "2012Dec" "2012Dec" "2013Mar" "2013Mar" "2013Mar" "2013Jun" "2013Jun" "2013Jun" "2013Sep" "2013Sep" "2013Sep" "2013Dec" "2013Dec" "2013Dec" "2014Mar"
[16] "2014Mar" "2014Mar" "2014Jun" "2014Jun" "2014Jun" "2014Sep" "2014Sep" "2014Sep" "2014Dec"