Dates and Times

🗓️ R Dates and Times: Your Time-Traveling Adventure!

Imagine you have a magic calendar that can remember any day ever—past, present, or future. In R, we have special tools to work with dates and times. Think of them as your personal time-keepers!

📅 What is a Date Class?

Think of the Date class like a simple wall calendar. It only shows days—no hours, no minutes, no seconds. Just the day!

The Date Class is Like a Birthday Tracker

Your birthday is just a day, right? You don’t say “I was born at exactly 3:42 PM.” You just say “March 15th.” That’s what the Date class does!

# Create a date - like marking a day on your calendar
my_birthday <- as.Date("2015-03-15")
print(my_birthday)
# [1] "2015-03-15"

Behind the scenes: R counts days from January 1, 1970. That’s day zero! Everything before is negative, everything after is positive.

# Day zero - the start of R's calendar
as.Date("1970-01-01")  # This is day 0!

# How many days since day zero?
as.numeric(as.Date("2024-01-01"))
# [1] 19723 (about 54 years of days!)

⏰ POSIXct and POSIXlt: The Precise Time-Keepers

Sometimes you need MORE than just the day. You need the exact moment—like when a race starts or when your favorite show begins!

POSIXct: The Stopwatch

POSIXct is like a super-precise stopwatch. It counts every single second since January 1, 1970.

# Create a precise moment in time
party_time <- as.POSIXct("2024-12-25 15:30:00")
print(party_time)
# [1] "2024-12-25 15:30:00"

# How many seconds since 1970?
as.numeric(party_time)
# [1] 1735137000 (lots of seconds!)

Why use POSIXct?

• It’s fast and simple
• Great for comparing times
• Takes up less space in memory

POSIXlt: The Time Detective

POSIXlt is like a detective’s notebook. It breaks time into pieces you can examine one by one!

# Create a time and break it into pieces
my_time <- as.POSIXlt("2024-07-04 09:30:45")

# Look at each piece!
my_time$year   # Years since 1900 (124 = 2024)
my_time$mon    # Month (0-11, so 6 = July)
my_time$mday   # Day of month (4)
my_time$hour   # Hour (9)
my_time$min    # Minutes (30)
my_time$sec    # Seconds (45)
my_time$wday   # Day of week (0=Sunday)

POSIXlt gives you:

• sec → Seconds (0-59)
• min → Minutes (0-59)
• hour → Hours (0-23)
• mday → Day of month (1-31)
• mon → Month (0-11, tricky!)
• year → Years since 1900
• wday → Weekday (0-6)
• yday → Day of year (0-365)

🎨 Creating and Parsing Dates

“Parsing” means teaching R to read a date written in different ways. Just like how “Dec 25” and “25/12” and “2024-12-25” all mean Christmas!

Creating Dates from Text

# The standard way (Year-Month-Day)
christmas <- as.Date("2024-12-25")

# From American format (Month/Day/Year)
independence <- as.Date("07/04/2024",
                        format = "%m/%d/%Y")

# From written words
new_year <- as.Date("January 1, 2025",
                    format = "%B %d, %Y")

The Magic Format Codes

Think of these as secret decoder symbols:

Parsing Times (POSIXct)

# Standard format
meeting <- as.POSIXct("2024-06-15 14:30:00")

# Custom format
party <- as.POSIXct("June 15, 2024 2:30 PM",
                    format = "%B %d, %Y %I:%M %p")

🎭 Date Formatting: Making Dates Look Pretty

Now let’s do the opposite—turn R dates into readable text for humans!

The format() Function

today <- Sys.Date()  # Gets today's date

# Different styles
format(today, "%B %d, %Y")
# "December 27, 2024"

format(today, "%m/%d/%y")
# "12/27/24"

format(today, "%A, %B %d")
# "Friday, December 27"

Formatting Times

now <- Sys.time()  # Gets current time

format(now, "%I:%M %p")
# "03:45 PM"

format(now, "%H:%M:%S")
# "15:45:30"

format(now, "%A at %I:%M %p")
# "Friday at 03:45 PM"

➕ Date Arithmetic: Time Math!

Here’s where dates become really fun—you can add, subtract, and compare them!

Adding and Subtracting Days

today <- Sys.Date()

# One week from now
next_week <- today + 7
print(next_week)

# Two weeks ago
two_weeks_ago <- today - 14
print(two_weeks_ago)

Finding the Difference Between Dates

start <- as.Date("2024-01-01")
end <- as.Date("2024-12-31")

# How many days between?
difference <- end - start
print(difference)
# Time difference of 365 days

Using difftime() for More Control

birthday <- as.Date("2024-03-15")
today <- as.Date("2024-12-27")

# Days until next birthday
difftime(birthday + 365, today, units = "days")

# Weeks between two dates
difftime(end, start, units = "weeks")
# About 52 weeks!

Time Arithmetic (POSIXct)

now <- Sys.time()

# Add 2 hours (in seconds!)
later <- now + (2 * 60 * 60)

# Add 30 minutes
half_hour <- now + (30 * 60)

# Difference in hours
meeting <- as.POSIXct("2024-12-27 18:00:00")
difftime(meeting, now, units = "hours")

🔍 Date Component Extraction: Taking Dates Apart

Sometimes you need just ONE piece of a date—like just the month or just the year.

Extracting from Date Objects

my_date <- as.Date("2024-07-04")

# Get individual parts using format()
format(my_date, "%Y")  # "2024" (year)
format(my_date, "%m")  # "07" (month)
format(my_date, "%d")  # "04" (day)
format(my_date, "%A")  # "Thursday" (weekday)
format(my_date, "%B")  # "July" (month name)

Extracting from POSIXlt (The Easy Way!)

my_time <- as.POSIXlt("2024-07-04 15:30:45")

# Direct access to parts!
my_time$year + 1900  # 2024 (add 1900!)
my_time$mon + 1      # 7 (add 1 for real month)
my_time$mday         # 4
my_time$hour         # 15
my_time$min          # 30
my_time$sec          # 45
my_time$wday         # 4 (Thursday, 0=Sunday)
my_time$yday         # 185 (day of year)

Quick Extraction Functions

today <- Sys.Date()
now <- Sys.time()

# Get the weekday (as number or name)
weekdays(today)   # "Friday"
months(today)     # "December"
quarters(today)   # "Q4"

🎯 Quick Reference Chart

graph TD
    A["R Date/Time"] --> B["Date Class"]
    A --> C["POSIXct"]
    A --> D["POSIXlt"]

    B --> E["Days only<br/>No time info"]
    C --> F["Seconds since 1970<br/>Compact & fast"]
    D --> G["Broken into pieces<br/>Easy to extract"]

    style A fill:#667eea,color:#fff
    style B fill:#4ECDC4,color:#fff
    style C fill:#FF6B6B,color:#fff
    style D fill:#95E1D3,color:#000

🌟 Remember This!

1. Date = Just days, like a calendar
2. POSIXct = Seconds since 1970, like a stopwatch
3. POSIXlt = Time broken into pieces, like a puzzle
4. format() = Turn dates into pretty text
5. as.Date() / as.POSIXct() = Create dates from text
6. Math works! = Add days, subtract dates, find differences

You’re now a time-traveler in R! 🚀