January 25, 4092

January 25, 4092 is a joke esoteric language that can only be compiled on the specific date January 25, 4092.
This is the only valid program:

CheckDate(01254092) => True = {
  Execute(main);
};

Interpreter

The following implementation is realized in Common Lisp:

(defstruct (Date
  (:constructor make-date (year month day)))
  "The Date class encapsulates a specific date, specified by its
   year, month, and day, in a Gregorian date."
  (year  (error "Missing year.")  :type (integer *  *) :read-only T)
  (month (error "Missing month.") :type (integer 1 12) :read-only T)
  (day   (error "Missing day.")   :type (integer 1 31) :read-only T))

(defun make-date-from-year-month-and-day (year month day)
  "Returns a fresh Date whose days are appropriated from the
   specified YEAR, MONTH, and DAY."
  (declare (type (integer *  *) year)
           (type (integer 1 12) month)
           (type (integer 1 31) day))
  (the Date (make-date year month day)))

(defun decode-date (timestamp)
  "Returns a fresh Date derived from the TIMESTAMP, this
   specifying the non-negative tally of seconds having elapsed since
   January 1st, 1900 GMT, at 00:00:00 of time."
  (declare (type (integer 0 *) timestamp))
  (multiple-value-bind
      (second minute hour day month year day-of-week
       uses-daylight-saving-time-p time-zone)
      (decode-universal-time timestamp)
    (declare (type (integer 0 59) second) (ignore second)
             (type (integer 0 59) minute) (ignore minute)
             (type (integer 0 23) hour)   (ignore hour)
             (type (integer 1 31) day)
             (type (integer 1 12) month)
             (type integer        year)
             (type (integer 0  6) day-of-week) (ignore day-of-week)
             (type boolean        uses-daylight-saving-time-p)
             (ignore              uses-daylight-saving-time-p)
             (type integer        time-zone) (ignore time-zone))
    (the Date (make-date year month day))))

(defun make-current-date ()
  "Returns a fresh Date initialized with the current date."
  (the Date (decode-date (get-universal-time))))

(defun equal-dates-p (first-date second-date)
  "Determines whether the FIRST-DATE and the SECOND-DATE specify
   the same point in time, returning on confirmation a boolean value
   of T, otherwise NIL."
  (declare (type Date first-date) (type Date second-date))
  (the boolean
    (not (null
      (and (= (date-year  first-date) (date-year  second-date))
           (= (date-month first-date) (date-month second-date))
           (= (date-day   first-date) (date-day   second-date)))))))

;;; -------------------------------------------------------

(declaim (type (simple-string 43) +MINIMAL-VALID-PROGRAM+))
(declaim (type Date               +VALID-COMPILATION-DATE+))

(defparameter +MINIMAL-VALID-PROGRAM+
  "CheckDate(01254092)=>True={Execute(main);};"
  "A valid \"January 25, 4092\" program without whitespaces.")

(defparameter +VALID-COMPILATION-DATE+
  (make-date-from-year-month-and-day 4092 1 25)
  "A \"January 25, 4092\" program's valid compilation date.")

;;; -------------------------------------------------------

(defun whitespace-character-p (candidate)
  "Determines whether the CANDIDATE represents a whitespace, returning
   on confirmation a boolean value of T, otherwise NIL."
  (declare (type character candidate))
  (the boolean
    (not (null
      (member candidate '(#\Space #\Tab #\Newline) :test #'char=)))))

(defun remove-whitespaces (source)
  "Expunges all whitespaces from the SOURCE and returns the result."
  (declare (type string source))
  (the string (remove-if #'whitespace-character-p source)))

(defun validate-program (source)
  "Determines the SOURCE's compliance with the sole
   \"January 25, 4092\" program format, returning no value on
   affirmation; otherwise signaling an error of an unspecified type."
  (declare (type string source))
  (unless (string= source +MINIMAL-VALID-PROGRAM+)
    (error "Invalid program: ~s." source))
  (values))

(defun validate-date (date)
  "Determines the DATE's validity for a \"January 25, 4092\" program's
   execution, returning no value on affirmation; otherwise signaling an
   error of an unspecified type."
  (declare (type Date date))
  (unless (equal-dates-p date +VALID-COMPILATION-DATE+)
    (error "Invalid compilation date: ~s." date))
  (values))

(defun interpret-January-25-4092 (code &key (simulated-date (make-current-date)))
  "Interprets the piece of \"January 25, 4092\" source CODE, optionally
   imputing the SIMULATED-DATE in lieu of the current one, and returns
   no value."
  (declare (type string code) (type Date simulated-date))
  (validate-program (remove-whitespaces code))
  (validate-date    simulated-date)
  (values))