Tutorial

Convert to Julian Day

Authors: Megan A. Jones, Marisa Guarinello, Courtney Soderberg, Leah A. Wasser

Last Updated: May 13, 2021

This tutorial defines Julian (year) day as most often used in an ecological context, explains why Julian days are useful for analysis and plotting, and teaches how to create a Julian day variable from a Date or Data/Time class variable.

Learning Objectives

After completing this tutorial, you will be able to:

Define a Julian day (year day) as used in most ecological contexts.
Convert a Date or Date/Time class variable to a Julian day variable.

Things You’ll Need To Complete This Tutorial

You will need the most current version of R and, preferably, RStudio loaded on your computer to complete this tutorial.

Install R Packages

lubridate: install.packages("lubridate")

More on Packages in R – Adapted from Software Carpentry.

Download Data

NEON Teaching Data Subset: Meteorological Data for Harvard Forest

The data used in this lesson were collected at the National Ecological Observatory Network's Harvard Forest field site. These data are proxy data for what will be available for 30 years on the NEON data portal for the Harvard Forest and other field sites located across the United States.

Download Dataset

Set Working Directory: This lesson assumes that you have set your working directory to the location of the downloaded and unzipped data subsets.

An overview of setting the working directory in R can be found here.

R Script & Challenge Code: NEON data lessons often contain challenges that reinforce learned skills. If available, the code for challenge solutions is found in the downloadable R script of the entire lesson, available in the footer of each lesson page.

Convert Between Time Formats - Julian Days

Julian days, as most often used in an ecological context, is a continuous count of the number of days beginning at Jan 1 each year. Thus each year will have up to 365 (non-leap year) or 366 (leap year) days.

**Data Note:** This format can also be called ordinal day or year day. In some contexts, Julian day can refer specifically to a numeric day count since 1 January 4713 BCE or as a count from some other origin day, instead of an annual count of 365 or 366 days.

Including a Julian day variable in your dataset can be very useful when comparing data across years, when plotting data, and when matching your data with other types of data that include Julian day.

Load the Data

Load this dataset that we will use to convert a date into a year day or Julian day.

Notice the date is read in as a character and must first be converted to a Date class.

# Load packages required for entire script
library(lubridate)  #work with dates

# set working directory to ensure R can find the file we wish to import
wd <- "~/Git/data/"

# Load csv file of daily meteorological data from Harvard Forest
# Factors=FALSE so strings, series of letters/ words/ numerals, remain characters
harMet_DailyNoJD <- read.csv(
  file=paste0(wd,"NEON-DS-Met-Time-Series/HARV/FisherTower-Met/hf001-06-daily-m-NoJD.csv"),
  stringsAsFactors = FALSE
  )

# what data class is the date column? 
str(harMet_DailyNoJD$date)

##  chr [1:5345] "2/11/01" "2/12/01" "2/13/01" "2/14/01" "2/15/01" ...

# convert "date" from chr to a Date class and specify current date format
harMet_DailyNoJD$date<- as.Date(harMet_DailyNoJD$date, "%m/%d/%y")

Convert with yday()

To quickly convert from from Date to Julian days, can we use yday(), a function from the lubridate package.

# to learn more about it type
?yday

We want to create a new column in the existing data frame, titled julian, that contains the Julian day data.

# convert with yday into a new column "julian"
harMet_DailyNoJD$julian <- yday(harMet_DailyNoJD$date)  

# make sure it worked all the way through. 
head(harMet_DailyNoJD$julian) 

## [1] 42 43 44 45 46 47

tail(harMet_DailyNoJD$julian)

## [1] 268 269 270 271 272 273

**Data Tip:** In this tutorial we converted from `Date` class to a Julian day, however, you can convert between any recognized date/time class (POSIXct, POSIXlt, etc) and Julian day using `yday`.

Get Lesson Code

Convert-to-Julian-Day-In-R.R