test(calendar-sync): add 68 tests across 13 files for untested pure functions

Covers core parsing (parse-ics-datetime, parse-timestamp, format-timestamp,
split-events, parse-event), date utilities (add-months, add-days,
weekday-to-number, date-weekday, event-start-time), and timezone
(format-timezone-offset, convert-utc-to-local, localize-parsed-datetime).

1 files changed, 43 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--add-months.el b/tests/test-calendar-sync--add-months.el
new file mode 100644
index 00000000..fdc00208
--- /dev/null
+++ b/tests/test-calendar-sync--add-months.el
@@ -0,0 +1,43 @@
+;;; test-calendar-sync--add-months.el --- Tests for month arithmetic -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--add-months. Pure date arithmetic.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--add-months-normal-forward ()
+  "Adding 3 months advances correctly."
+  (should (equal '(2026 6 15) (calendar-sync--add-months '(2026 3 15) 3))))
+
+(ert-deftest test-calendar-sync--add-months-normal-backward ()
+  "Subtracting months goes backward."
+  (should (equal '(2025 12 15) (calendar-sync--add-months '(2026 3 15) -3))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--add-months-boundary-year-rollover ()
+  "Adding months past December rolls into next year."
+  (should (equal '(2027 2 1) (calendar-sync--add-months '(2026 11 1) 3))))
+
+(ert-deftest test-calendar-sync--add-months-boundary-year-rollback ()
+  "Subtracting months past January rolls into previous year."
+  (should (equal '(2025 11 1) (calendar-sync--add-months '(2026 2 1) -3))))
+
+(ert-deftest test-calendar-sync--add-months-boundary-zero ()
+  "Adding zero months returns same date."
+  (should (equal '(2026 3 15) (calendar-sync--add-months '(2026 3 15) 0))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--add-months-error-large-offset ()
+  "Adding 24 months (2 years) works correctly."
+  (should (equal '(2028 3 15) (calendar-sync--add-months '(2026 3 15) 24))))
+
+(provide 'test-calendar-sync--add-months)
+;;; test-calendar-sync--add-months.el ends here