EDS 221 Day 8 PM

Dates and times

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August 19^th, 2026

Why dates are tricky

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Dates look like plain text, but they aren’t — "01/02/2017" could mean January 2^nd or February 1^st depending on where you’re reading from.

library(tidyverse)
library(lubridate)

class("2017-01-31")

[1] "character"

A character string has no concept of “one day later.” A proper date type does.

Parsing dates: ymd(), mdy(), dmy()

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lubridate’s parsing functions are named for the order the year, month, and day appear in your text — they all return the same kind of date.

ymd("2017-01-31")

[1] "2017-01-31"

mdy("January 31st, 2017")

[1] "2017-01-31"

dmy("31-Jan-2017")

[1] "2017-01-31"

Parsing datetimes: ymd_hms()

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Add _hms, _hm, or _h to also capture a time of day. Unspecified time components default to zero.

ymd_hms("2017-01-31 20:11:59")

[1] "2017-01-31 20:11:59 UTC"

mdy_hm("01/31/2017 08:01")

[1] "2017-01-31 08:01:00 UTC"

Extracting components

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Once something is a date, you can pull out any piece of it — useful for grouping and filtering.

release_date <- ymd("2026-08-19")

year(release_date)

[1] 2026

month(release_date, label = TRUE)

[1] Aug
12 Levels: Jan < Feb < Mar < Apr < May < Jun < Jul < Aug < Sep < ... < Dec

wday(release_date, label = TRUE)

[1] Wed
Levels: Sun < Mon < Tue < Wed < Thu < Fri < Sat

Date arithmetic

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Dates support arithmetic directly. days(), weeks(), and friends add a fixed span of calendar time.

start <- ymd("2026-08-19")

start + days(1)

[1] "2026-08-20"

start + weeks(2)

[1] "2026-09-02"

ymd("2026-12-25") - start

Time difference of 128 days

The last line returns a difftime — the number of days between two dates.

What to memorize

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Dates and datetimes are a distinct type from text — parse early, and let lubridate handle the arithmetic.

Creating

• ymd() (and permutations)
• ymd_hms() (and permutations)

Components

• year(), month(), day(), wday()
• hour(), minute(), second()