Calc: fix formatting and parsing Unix time (bug#43759)

The number of days from epoch to Jan 1, 1970 that was used in parsing
and formatting Unix time was incorrect.  The previous fix
(in e368697ce36) was incomplete.

Reported by Vincent Belaïche.

* lisp/calc/calc-forms.el (math-unix-epoch): New constant.
(math-format-date-part, math-parse-standard-date, calcFunc-unixtime):
Use math-unix-epoch instead of a constant that is sometimes wrong.
* test/lisp/calc/calc-tests.el (calc-unix-date): New test.

1 files changed, 40 insertions, 0 deletions

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