Fix handling of delta values with negative month field

* lisp/calendar/time-date.el (decoded-time-add): If the new
variable is less then zero, the year needs to be decremented
by quotient of new and 12 increased by one.
* test/lisp/calendar/time-date-tests.el (test-decoded-add):
Add applicable test cases.  (Bug#68969)

1 files changed, 12 insertions, 0 deletions

diff --git a/test/lisp/calendar/time-date-tests.el b/test/lisp/calendar/time-date-tests.el
index b8d3381528e..7df1e1b0da7 100644
--- a/test/lisp/calendar/time-date-tests.el
+++ b/test/lisp/calendar/time-date-tests.el
@@ -161,6 +161,18 @@
     (should (equal (decoded-time-add time (mdec :month 10))
                    '(12 15 16 8 5 2020 1 t 7200)))
 
+    (should (equal (decoded-time-add time (mdec :month -1))
+                   '(12 15 16 8 6 2019 1 t 7200)))
+
+    (should (equal (decoded-time-add time (mdec :month -10))
+                   '(12 15 16 8 9 2018 1 t 7200)))
+
+    (should (equal (decoded-time-add time (mdec :month -14))
+                   '(12 15 16 8 5 2018 1 t 7200)))
+
+    (should (equal (decoded-time-add time (mdec :month -24))
+                   '(12 15 16 8 7 2017 1 t 7200)))
+
     (should (equal (decoded-time-add time (mdec :day 1))
                    '(12 15 16 9 7 2019 1 t 7200)))