diff --git a/lisp/calc/calc-forms.el b/lisp/calc/calc-forms.el
index 6d70126c098..a2f66968665 100644
--- a/lisp/calc/calc-forms.el
+++ b/lisp/calc/calc-forms.el
@@ -709,6 +709,10 @@ as measured in the number of days before December 31, 1 BC (Gregorian).")
   "The beginning of the Julian date calendar,
 as measured in the integer number of days before December 31, 1 BC (Gregorian).")
 
+(defconst math-unix-epoch 719163
+  "The beginning of Unix time: days from December 31, 1 BC (Gregorian)
+to Jan 1, 1970 AD.")
+
 (defun math-format-date-part (x)
   (cond ((stringp x)
 	 x)
@@ -730,7 +734,8 @@ as measured in the integer number of days before December 31, 1 BC (Gregorian)."
                               (math-floor math-fd-date)
                               math-julian-date-beginning-int)))
 	((eq x 'U)
-	 (math-format-number (nth 1 (math-date-parts math-fd-date 719164))))
+	 (math-format-number (nth 1 (math-date-parts math-fd-date
+                                                     math-unix-epoch))))
         ((memq x '(IYYY Iww w))
          (progn
            (or math-fd-iso-dt
@@ -1173,7 +1178,7 @@ as measured in the integer number of days before December 31, 1 BC (Gregorian)."
 		      (setq num (math-match-substring math-pd-str 0)
 			    math-pd-str (substring math-pd-str (match-end 0))
 			    num (math-date-to-dt
-				 (math-add 719164
+				 (math-add math-unix-epoch
 					   (math-div (math-read-number num)
 						     '(float 864 2))))
 			    hour (nth 3 num)
@@ -1434,11 +1439,11 @@ as measured in the integer number of days before December 31, 1 BC (Gregorian)."
 (defun calcFunc-unixtime (date &optional zone)
   (if (math-realp date)
       (progn
-	(setq date (math-add 719163 (math-div date '(float 864 2))))
+	(setq date (math-add math-unix-epoch (math-div date '(float 864 2))))
 	(list 'date (math-sub date (math-div (calcFunc-tzone zone date)
 					     '(float 864 2)))))
     (if (eq (car date) 'date)
-	(math-add (nth 1 (math-date-parts (nth 1 date) 719163))
+	(math-add (nth 1 (math-date-parts (nth 1 date) math-unix-epoch))
 		  (calcFunc-tzone zone date))
       (math-reject-arg date 'datep))))
 
diff --git a/test/lisp/calc/calc-tests.el b/test/lisp/calc/calc-tests.el
index 4dded007f79..0df96a0e2db 100644
--- a/test/lisp/calc/calc-tests.el
+++ b/test/lisp/calc/calc-tests.el
@@ -534,6 +534,46 @@ An existing calc stack is reused, otherwise a new one is created."
       )
   ))
 
+(ert-deftest calc-unix-date ()
+  (let* ((d-1970-01-01 (math-parse-date "1970-01-01"))
+         (d-2020-09-07 (math-parse-date "2020-09-07"))
+         (d-1991-01-09-0600 (math-parse-date "1991-01-09 06:00")))
+    ;; calcFunc-unixtime (command "t U") converts a date value to Unix time,
+    ;; and a number to a date.
+    (should (equal d-1970-01-01 '(date 719163)))
+    (should (equal (calcFunc-unixtime d-1970-01-01 0) 0))
+    (should (equal (calc-tests--calc-to-number (cadr (calcFunc-unixtime 0 0)))
+                   (cadr d-1970-01-01)))
+    (should (equal (calcFunc-unixtime d-2020-09-07 0)
+                   (* (- (cadr d-2020-09-07)
+                         (cadr d-1970-01-01))
+                      86400)))
+    (should (equal (calcFunc-unixtime d-1991-01-09-0600 0)
+                   663400800))
+    (should (equal (calc-tests--calc-to-number
+                    (cadr (calcFunc-unixtime 663400800 0)))
+                   726841.25))
+
+    (let ((calc-date-format '(U)))
+      ;; Test parsing Unix time.
+      (should (equal (calc-tests--calc-to-number
+                      (cadr (math-parse-date "0")))
+                     719163))
+      (should (equal (calc-tests--calc-to-number
+                      (cadr (math-parse-date "469324800")))
+                     (+ 719163 (/ 469324800 86400))))
+      (should (equal (calc-tests--calc-to-number
+                      (cadr (math-parse-date "663400800")))
+                     726841.25))
+
+      ;; Test formatting Unix time.
+      (should (equal (math-format-date d-1970-01-01) "0"))
+      (should (equal (math-format-date d-2020-09-07)
+                     (number-to-string (* (- (cadr d-2020-09-07)
+                                             (cadr d-1970-01-01))
+                                          86400))))
+      (should (equal (math-format-date d-1991-01-09-0600) "663400800")))))
+
 (provide 'calc-tests)
 ;;; calc-tests.el ends here