• PHP

$formattedVar = date("Y-m-d", strtotime($unformattedVar));

# Take the unformated Variable, let PHP do it's best to
# 'understand' it and then format the resultant into the 
# desired format.

• PHP

$x = date("Y-m-d", strtotime($d));

• Ada

X : constant String :=
    Ada.Calendar.Formatting.Image (D) (1 .. 10);

• Clojure

(def x (.format (java.text.SimpleDateFormat. "yyyy-MM-dd") d))

• Cobol

IDENTIFICATION DIVISION.
PROGRAM-ID. date format.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 curr-date.
   03 yyyy      pic 9(4).
   03 mm        pic 9(2).
   03 dd        pic 9(2).
01 d.
   03 year      pic 9(4). 
   03 FILLER    pic x VALUE '-'.
   03 month     pic 99. 
   03 FILLER    pic x VALUE '-'.
   03 day       pic 99. 
PROCEDURE DIVISION.
   MOVE FUNCTION CURRENT-DATE TO curr-date
   MOVE yyyy to year
   MOVE mm   to month
   MOVE dd   to day
   DISPLAY d 
STOP RUN.

• C++

int main()
{
	char x[32]{};
	time_t a = time(nullptr);
	struct tm d;
	if (localtime_s(&d, &a) == 0) {
		strftime(x, sizeof(x), "%F", &d);
		std::cout << x << std::endl;
	}

	return 0;

}

• C#

string x = d.ToString("yyyy-MM-dd");

• D

string x = Date(1993, 10, 26).toISOExtString;

• Dart

x = DateFormat('YYYY-MM-DD').format(d);

• Elixir

x = Date.to_iso8601(d)

• Erlang

D = erlang:localtime(),
{{Year, Month, Day}, {_Hour, _Minute, _Second}} = D,
X = lists:flatten(io_lib:format("~4..0w-~2..0w-~2..0w", [Year, Month, Day])).  % "2017-07-02"

• Fortran

program p
integer, dimension(8) :: d
character(len=10) :: x
call date_and_time (values=d)
write(x,'(i4.4,"-",i2.2,"-",i2.2)')d(1),d(2),d(3)
print *,'DATE=',x
end program p

• Go

x := d.Format("2006-01-02")

• Haskell

x = showGregorian d

• JS

let x = d.toISOString().slice(0, 10)

• JS

let y = d.getFullYear(),
    m = d.getMonth() + 1,
    D = d.getDate(), x
m = m.toString().padStart(2, '0')
D = D.toString().padStart(2, '0')
x = `${y}-${m}-${D}`

• Java

String x = String.format("%1$tY-%1$tm-%1$td", d)

• Java

String x = new SimpleDateFormat("yyyy-MM-dd").format(d);

• Java

Date d = getInstance().getTime();
String s = "%tY-%<tm-%<td".formatted(d);

• Lua

x = os.date("%F",d)

• Obj-C

// once
static NSDateFormatter *df; 
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
  df=[[NSDateFormatter alloc] init];
  df.locale=[[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
  df.timeZone=[NSTimeZone timeZoneForSecondsFromGMT:0];
  df.dateFormat=@"yyyy-MM-dd";
});
// then, wherever needed
NSString *x=[df stringFromDate:d];

• Pascal

DefaultFormatSettings.ShortDateFormat := 'yyyy-mm-dd';
X := DateToStr(D);

• Perl

$x = strftime "%Y-%m-%d", localtime($d);

• Python

d = date.today()
x = d.isoformat()

• Python

d = date(2016, 9, 28)
x = d.strftime('%Y-%m-%d')

• Ruby

d = Date.today
x = d.to_s

• Rust

let format = format_description!("[year]-[month]-[day]");
let x = d.format(&format).expect("Failed to format the date");

• Rust

Utc::today().format("%Y-%m-%d")

• Smalltalk

x := d yyyymmdd.

• VB

Dim x As String = d.ToString("yyyy-MM-dd")