R Type

Date

"Date" is a special data class for date values in R. Strictly saying, Date is not an R Type, but a class. A class is an attribute assigned to an object regardless of its internal storage structure. To see this, execute the following code

x <- as.Date("2022-11-03")
class(x)  # "Date"
typeof(x) # "double"

The only correct date format: YYYY-MM-DD

Conversion to Date

Use as.Date(x) to convert to Date class
- You need to specify the format of x if it is not in YYYY-MM-DD
  - as.Date("1/12/2019", format="%m/%d/%Y")
  - Pay attention to the format: wrong format may give the wrong date
    - as.Date("1/12/2019", format="%m/%d/%y") gives "2020-01-12" because %y is for a two-digit year, and is replaced by 20 here
  - See #Conversion Specification for the conversion specification reference
A similar function: reader::parse_date
- parse_date will return NA for parse_date("1/12/2019", format="%m/%d/%y")
Use package lubridate

Conversion Specification

Conversion specification	Description	Example
%A	Full weekday	Sunday, Thursday
%b or %h	Abbreviated month	May, Jul
%B	Full month	May, July
%d	Day of the month (01-31)	27, 07
%j	Day of the year (001-366)	148, 188
%m	Month (01-12)	05, 07
%U	Week (01-53) with Sunday as the first day of the week	22, 27
%w	Weekday (0-6) with Sunday as 0	0, 4
%W	Week (00-53) with Monday as the first day of the week	21, 27
%x	Date, locale-specific
%y	Year without century (00-99)	84, 05
%Y	Year with century	1984, 2005
%C	Century	19, 20
%D	Date formatted %m/%d/%y	05/27/84, 07/07/05
%u	Weekday (1-7) Monday is 1	7, 4
%n	Newline on output or arbitrary whitespace on input
%t	Tab on output or arbitrary whitespace on input

See ?strptime for the reference inside R.

Date Operation

Date class supports the following operations

# Subtraction
as.Date("2022-11-01") - as.Date("2020-11-01")
# Time difference of 730 days 

# Addition
as.Date("2022-11-01") + 7
# "2022-11-08"

# Comparison
as.Date("2022-11-01") < as.Date("2020-11-01")
# False

scale_x_date

Since dates can be calculated and compared, it is easy to specify the limits and breaks in ggplot2.

base + sacle_x_date(limits = c(ymd("2022-01-01"), ymd("2022-11-03")))
base + sacle_x_date(limits = c(Sys.Date() - 19, NA))
base + sacle_x_date(breaks = "1 week"))
base + scale_x_date(date_labels = "%b %d")

Other Functions

Sys.date() returns today's date
weekdays(), months(), and quarters() return the weekday, month, and quarter of the date respectively

by zcysxy