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diff --git a/tests/test-calendar-sync--add-months.el b/tests/test-calendar-sync--add-months.el
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+;;; 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