R Date and Time

Dates and times are a special data type in R. They look like strings ("2024-08-15") but behave like numbers — you can subtract two dates to get the number of days between them, add days to a date, and sort a dataset chronologically. R has three main date and time classes.

Date and Time Classes in R

Class       Stores                   Example
──────────────────────────────────────────────────────────
Date        Date only               "2024-08-15"
POSIXct     Date + Time (numeric)   "2024-08-15 14:30:00"
POSIXlt     Date + Time (list)      has named parts (sec, min, hour...)

Creating Date Values

today <- Sys.Date()         # today's date
print(today)                # "2024-08-15"
class(today)                # "Date"

# From string
dob <- as.Date("1995-06-20")          # ISO format (default)
event <- as.Date("15/08/2024", format="%d/%m/%Y")  # custom format

Date Format Codes

Code    Meaning           Example
───────────────────────────────────────────
%Y      4-digit year       2024
%y      2-digit year       24
%m      Month (01-12)      08
%B      Full month name    August
%b      Abbrev month       Aug
%d      Day (01-31)        15
%A      Full weekday       Thursday
%a      Abbrev weekday     Thu

Date Arithmetic

today    <- as.Date("2024-08-15")
birthday <- as.Date("2024-10-05")

birthday - today         # 51 days
today + 30               # "2024-09-14"
today - 365              # "2023-08-15"

# Days between two dates
diff_days <- as.numeric(birthday - today)
cat("Days until birthday:", diff_days, "\n")

Extracting Date Parts

d <- as.Date("2024-08-15")

format(d, "%Y")    # "2024"  year
format(d, "%m")    # "08"    month
format(d, "%d")    # "15"    day
format(d, "%A")    # "Thursday"  weekday
weekdays(d)        # "Thursday"
months(d)          # "August"
quarters(d)        # "Q3"

Date Sequences

# Daily sequence
seq(as.Date("2024-01-01"), as.Date("2024-01-07"), by="day")

# Monthly sequence
seq(as.Date("2024-01-01"), as.Date("2024-12-01"), by="month")

# Weekly
seq(as.Date("2024-01-01"), by="week", length.out=6)

Working with Date and Time (POSIXct)

now <- Sys.time()
print(now)
# "2024-08-15 14:30:45 IST"

# Create from string
event_time <- as.POSIXct("2024-08-15 09:00:00",
                           tz = "Asia/Kolkata")

# Extract parts
format(event_time, "%H:%M")    # "09:00"
format(event_time, "%I %p")    # "09 AM"

# Time difference
end   <- as.POSIXct("2024-08-15 17:30:00")
start <- as.POSIXct("2024-08-15 09:00:00")
difftime(end, start, units="hours")   # 8.5 hours

Practical: Age Calculator

birth_date  <- as.Date("1990-05-22")
today       <- Sys.Date()

years <- as.numeric(today - birth_date) %/% 365
cat("Age:", years, "years\n")

lubridate Package (Recommended)

library(lubridate)

ymd("2024-08-15")          # Date from YYYY-MM-DD
dmy("15-08-2024")          # Date from DD-MM-YYYY
mdy("08/15/2024")          # Date from MM-DD-YYYY

year(today)                # 2024
month(today)               # 8
day(today)                 # 15
wday(today, label=TRUE)    # "Thu"

today %m+% months(3)       # add 3 months safely
today %m-% years(1)        # subtract 1 year

Date and time handling appears in virtually every real dataset — transaction logs, sensor readings, survey timestamps, and sales records all carry date information. Mastering date arithmetic, formatting, and extraction lets you create time-based analyses like monthly trends, duration calculations, and age groupings.