STAT 118: Notes R

Working with dates using lubridate

Author

Affiliation

Emily Malcolm-White

Middlebury College

Date Formats

Think of how many different formats you know of to format a date:

• 2023 07 06
• Wed, Jun 7, 2023
• 07-06-23
• 06-07-23 14:55 ET
• 06/07/2023 2:55pm

Yikes!

Date, Time, and Datetime

Date/time data are data that conveys information about, you guessed it, date and/or time! There are three relevant data types when we talk about date/time data:

1. Date - only has the date (e.g. 2020-05-15)
2. Time - only has the time (e.g. 20:45:00)
3. Datetime - has both the date and time (e.g. 2020-05-15 20:45:00)

Lubridate

Artwork by @allisonhorst

#LOAD PACKAGES 
library(tidyverse)
library(lubridate)

Standard Date Format

The ymd() function transforms data in all kinds of different formats into a standardized date format displaying year, then month, then day.

ymd("06 02 04")

[1] "2006-02-04"

ymd("06/02/04")

[1] "2006-02-04"

ymd("20060204")  # works as well

[1] "2006-02-04"

ymd("2006 2 4")

[1] "2006-02-04"

ymd(060204)  # works with numbers

[1] "2006-02-04"

mdy() (month day year) and dmy() (day month year) formats also exist.

ymd_hms("2020-04-01 10:30:13")

[1] "2020-04-01 10:30:13 UTC"

ymd_hm("2020/04/01 10.30")

[1] "2020-04-01 10:30:00 UTC"

Solar Data

Shoal Marine Lab (SML) is a remote field station located on Appledore Island, Maine jointly operated by Cornell University and the University of New Hampshire. The island is powered primarily by solar power.

#read in Data
power <- read.csv("power.csv")

What format is the date in?

Warning

What happens if we try to make a line plot with the date in this format?

power %>% 
  ggplot(aes(x=Date, y=power_kW)) +
  geom_line()

`geom_line()`: Each group consists of only one observation.
ℹ Do you need to adjust the group aesthetic?

Yikes!

We need to put it in standardized date format first:

power <- power %>% 
  mutate(Date = mdy_hm(Date))  

Standardized Format helps us to create time series plots very easily!

power %>% 
  ggplot(aes(x=Date, y=power_kW)) +
  geom_line()

Picking out information

Artwork by @allisonhorst

Sometimes we need to pick out year, month, date so we can filter, sort, etc.

power <- power %>% 
  mutate(Year = year(Date)) %>%   
  mutate(Month = month(Date)) %>% 
  mutate(Day = day(Date))

Suppose we only want the time series plot for June 1:

power %>% 
  filter(Month == "6") %>%
  filter(Day == "1") %>% 
  ggplot(aes(x=Date, y=power_kW)) +
  geom_line()

Portal Data

The Portal Project is a long-term ecological study being conducted near Portal, AZ. Since 1977, the site has been used to study the interactions among rodents, ants and plants and their respective responses to climate.

#LOAD DATA
portal_rodent <- read.csv("https://github.com/weecology/PortalData/raw/main/Rodents/Portal_rodent.csv")

Unfortunately, because the information about datetime is divided up into different columns, R does not recognize it as date/time data. What we need to do is combine and convert all of these columns into datetime. To do this, we can use the function make_datetime()

portal_rodent <- portal_rodent %>% 
  mutate(datetime = make_datetime(year, month, day))