The code

Straight from labs/course-6-r/11_dates.R, unchanged.

# =====================================================================
# Run with R 4.3.3 (Rscript --vanilla). What it prints, and the plots it
# draws, are on the lab page, and tools/data-science/run_r_equivalents.py
# runs it again. (Until October 2026 R could not be installed where these
# labs are checked, so this file was desk-checked only; every number in its
# comments has since been checked against R's own output.)
# =====================================================================
# Experiment 11: Working with dates and times
# Python equivalent: python/11_dates.py

# --- BASE R ---
# Step 1: Make dates in base R, and format them
d <- as.Date("2026-08-26")                       # ISO is the safe default
d2 <- as.Date("26/08/2026", format = "%d/%m/%Y")
Sys.Date(); Sys.time()

format(d, "%d-%m-%Y")      # "26-08-2026"
format(d, "%d %B %Y")      # "26 August 2026"
weekdays(d); months(d)

# Step 2: Do arithmetic on dates
d + 30                     # date arithmetic works directly
difftime(as.Date("2026-12-25"), d, units = "days")   # 121 days

# --- lubridate: much easier ---
# Step 3: Do the same with lubridate
library(lubridate)
ymd("2026-08-26"); dmy("26-08-2026"); mdy("08-26-2026")
year(d); month(d); day(d); wday(d, label = TRUE)
d + days(30); d + months(1); d + years(1)
d %m+% months(1)     # SAFE month addition
# 31 Jan %m+% months(1) gives 28 Feb, not an invalid 31 Feb.

# --- WHY DATES MUST NOT BE STORED AS TEXT ---
# Step 4: Sort dates as text and as dates
as_text <- c("10/01/2026", "02/01/2026", "21/12/2025")
sort(as_text)
#   "02/01/2026" "10/01/2026" "21/12/2025"
#   ALPHABETICAL: the 2025 date sorts LAST. This is wrong and silent.

as_dates <- dmy(as_text)
sort(as_dates)
#   "2025-12-21" "2026-01-02" "2026-01-10"   <- correct chronological order

# Same lesson as Course 5: a date column stored as VARCHAR sorts and compares
# alphabetically, which is almost never what you want.

# --- FORMAT CODES (also used by format() in base R) ---
#   %Y 4-digit year   %y 2-digit year   %m month number
#   %B full month     %b abbreviated    %d day of month
#   %A full weekday   %a abbreviated    %H:%M:%S time

Where this sits

The same thing in Python: Working with Dates and Times in Python.

The theory behind it is in this course’s units; the whole lab, with all 18 experiments, is on the lab page.