Convert a Date Vector into Julian Day in R

Convert a date vector into Julian day in R

Try the following to convert from class character(i.e. text) to class POSIXlt, and then extract Julian day (yday):

tmp <- as.POSIXlt("16Jun10", format = "%d%b%y")
tmp$yday
# [1] 166

For more details on function settings:

?POSIXlt
?DateTimeClasses

Another option is to use a Date class, and then use format to extract a julian day (notice that this class define julian days between 1:366, while POSIXlt is 0:365):

tmp <- as.Date("16Jun10", format = "%d%b%y")
format(tmp, "%j")
# [1] "167"

Converting time vectors into Julian days

The second column seems to be the number of seconds since the current day started. 24 hours / day * 3600 seconds per hour is 86400 seconds per day. So SecondColumn/86400 gives the decimals of the Julian date. The second columns doesn't hold information on the date itself.

Julian day to date vector

That is interesting puzzle. We know that as.POSIXlt contains the day of the year number and that some date libraries convert to it, but I could not immediately find a parser that dealt with it.

Then again, date arithmentic is all we need. We always get the date of January 1. And the desired date is then simply the Jan 1 plus the 'day-of-year' number minus 1.

Code

yearyearday <- function(yr, yd) {
    base <- as.Date(paste0(yr, "-01-01")) # take Jan 1 of year
    day <- base + yd - 1
}

set.seed(42)  # make it reproducible
sample <- data.frame(year=1980:2020, doy=as.integer(rnorm(41,mean=91.1,sd=9.65)))

sample$date <- yearyearday(sample$year, sample$doy)

head(sample)

Output

R> yearyearday <- function(yr, yd) {
+     base <- as.Date(paste0(yr, "-01-01")) # take Jan 1 of year
+     day <- base + yd - 1
+ }
R> 
R> set.seed(42)  # make it reproducible
R> sample <- data.frame(year=1980:2020, 
+                       doy=as.integer(rnorm(41, mean=91.1, sd=9.65)))
R> 
R> sample$date <- yearyearday(sample$year, sample$doy)
R> 
R> head(sample)
  year doy       date
1 1980 104 1980-04-13
2 1981  85 1981-03-26
3 1982  94 1982-04-04
4 1983  97 1983-04-07
5 1984  95 1984-04-04
6 1985  90 1985-03-31
R> 

As so often with date calculation, nothings besides base R is needed.

Join year and julian day as a date column in R and plot it

To convert a Julian date to a Date object use the %j format. For example:

temp <- rep(runif(730,15,40))
doy <- rep(c(1:365),2)
year <- c("2015","2016")

jdate<-as.Date(paste(year, doy, sep="-"),"%Y-%j") 
df<- data.frame(jdate, temp)

library(ggplot2)
f<-ggplot(df, aes(x=jdate, y=temp))
f+ geom_point() + scale_x_date(date_breaks="6 week", date_labels="%Y-%j")

Generate first and last Julian day of each month

Call your date sequence of the firsts of each month starts. Then define ends = starts - 1. Then the julian days are format(c(starts, ends), "%j"). Use 2002 for the sequence so that the previous year isn't a leap year.

    starts = seq(as.Date("2002/1/1"), by = "month", length.out = 12)
    ends = starts - 1
    x = as.numeric(format(c(starts, ends), "%j"))
    sort(x)
    #  [1]   1  31  32  59  60  90  91 120 121 151 152 181 182 212 213 243 244 273 274 304 305
    # [22] 334 335 365

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