Error out if 'date-days-in-month' is given an invalid month

* lisp/calendar/time-date.el (date-days-in-month): Add test for
month validity; signal an error if it isn't.  (Bug#40217)
* test/lisp/calendar/time-date-tests.el (test-days-in-month): Add
a test for the new error.

2 files changed, 4 insertions, 1 deletions

diff --git a/lisp/calendar/time-date.el b/lisp/calendar/time-date.el
index e2402de8010..9b58a4884bc 100644
--- a/lisp/calendar/time-date.el
+++ b/lisp/calendar/time-date.el
@@ -355,6 +355,8 @@ is output until the first non-zero unit is encountered."
 
 (defun date-days-in-month (year month)
   "The number of days in MONTH in YEAR."
+  (unless (<= 1 month 12)
+    (error "Month %s invalid" month))
   (if (= month 2)
       (if (date-leap-year-p year)
           29
diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index 4c8f18a7a95..9c90300cfe6 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -31,7 +31,8 @@
 (ert-deftest test-days-in-month ()
   (should (= (date-days-in-month 2004 2) 29))
   (should (= (date-days-in-month 2004 3) 31))
-  (should-not (= (date-days-in-month 1900 3) 28)))
+  (should-not (= (date-days-in-month 1900 3) 28))
+  (should-error (date-days-in-month 2020 15)))
 
 (ert-deftest test-ordinal ()
   (should (equal (date-ordinal-to-time 2008 271)