← SASnRExtract Day, Month, Year, Weekday from Date

Introduction

Clinical reports frequently need pieces of a date — the day, the month, the year, or the name of the weekday — rather than the whole date at once.
Typical uses: group by month, band by year of enrolment, flag weekend visits, build custom date strings.
SAS offers the date-part functions day(), month(), year(), and weekday() — each takes a SAS date and returns an integer.
Tidyverse uses the lubridate package: day(), month(), year(), and wday(), with wday(..., label = TRUE) giving the weekday name.
Base R uses format() with %d, %m, %Y to pull out components and weekdays() for the weekday name.
This lesson extracts day, month, year, and weekday from a short vector of birth dates so each result is easy to compare.

Extract Day, Month, Year, Weekday from Date

SAS (Base SAS)+

<div style="font-family: Courier New; font-size: 14pt">
<b><span style="color:#000080;background-color:#ffffff;">data</span></b><span style="color:#000000;background-color:#ffffff;">&nbsp;dates;<br></span>
<span style="color:#000000;background-color:#ffffff;">&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color:#0000ff;background-color:#ffffff;">input</span><span style="color:#000000;background-color:#ffffff;">&nbsp;dob&nbsp;:&nbsp;</span><span style="color:#008080;background-color:#ffffff;">date9.</span>;
<span style="color:#000000;background-color:#ffffff;"><br>&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color:#0000ff;background-color:#ffffff;">format</span><span style="color:#000000;background-color:#ffffff;">&nbsp;dob&nbsp;</span><span style="color:#008080;background-color:#ffffff;">date9.</span>;
<span style="color:#000000;background-color:#ffffff;"><br>&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color:#0000ff;background-color:#ffffff;">datalines</span>;
<span style="color:#000000;background-color:#ffffff;"><br></span><span style="color:#000000;background-color:#ffffc0;">01JAN2000&nbsp;<br>15FEB1995&nbsp;<br>20DEC2010&nbsp;<br></span>;
<span style="color:#000000;background-color:#ffffff;"><br></span><b><span style="color:#000080;background-color:#ffffff;">run</span></b>;
<span style="color:#000000;background-color:#ffffff;"><br><br></span><b><span style="color:#000080;background-color:#ffffff;">data</span></b><span style="color:#000000;background-color:#ffffff;">&nbsp;extract_parts;<br></span>
<span style="color:#000000;background-color:#ffffff;">&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color:#0000ff;background-color:#ffffff;">set</span><span style="color:#000000;background-color:#ffffff;">&nbsp;dates;<br>&nbsp;&nbsp;&nbsp;&nbsp;day&nbsp;=&nbsp;day(dob);<br>&nbsp;&nbsp;&nbsp;&nbsp;month&nbsp;=&nbsp;month(dob);<br>&nbsp;&nbsp;&nbsp;&nbsp;year&nbsp;=&nbsp;year(dob);<br>&nbsp;&nbsp;&nbsp;&nbsp;weekday&nbsp;=&nbsp;weekday(dob);<br></span>
<b><span style="color:#000080;background-color:#ffffff;">run</span></b><span style="color:#000000;background-color:#ffffff;">;</span></div>

In SAS, each date part has its own function: day(dob), month(dob), year(dob), weekday(dob).
The input must be a SAS date value (a number of days since 1960-01-01); input dob : date9.; uses the DATE9 informat to parse strings like 01JAN2000.
Applying format dob date9.; makes the raw numeric date display as a human-readable date string.
day(dob) returns an integer 1-31; month(dob) returns 1-12; year(dob) returns the four-digit year.
weekday(dob) returns 1 for Sunday through 7 for Saturday — note this differs from the ISO convention where Monday is 1.
We run and inspect EXTRACT_PARTS: confirm the derived day, month, year, and weekday match the input dates.

R (tidyverse)+

library(tidyverse)
library(lubridate)

dates <- tribble(
  ~dob,
  "2000-01-01",
  "1995-02-15",
  "2010-12-20"
) %>%
  mutate(dob = ymd(dob)) %>%
  mutate(
    day = day(dob),
    month = month(dob),
    year = year(dob),
    weekday = wday(dob, label = TRUE)
  )

In tidyverse, the lubridate package gives us a clean set of extractor functions that match the SAS names closely.
ymd(dob) parses ISO-style date strings ("2000-01-01") into proper Date objects — the first step before any extraction.
day(dob), month(dob), year(dob) return integers for the corresponding component.
wday(dob, label = TRUE) returns a factor with weekday names (Sun, Mon, ..., Sat) — convenient for plotting or report layout.
We can drop label = TRUE if the numeric weekday is needed instead — lubridate uses Sunday = 1 by default, matching SAS.
For a different calendar convention (Monday = 1), pass week_start = 1 to wday().
We run and compare the extracted columns against the SAS output — values should match row for row.

Happy learning!

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Introduction

Extract Day, Month, Year, Weekday from Date