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diff --git a/tests/test-calendar-sync--format-timezone-offset.el b/tests/test-calendar-sync--format-timezone-offset.el
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+;;; test-calendar-sync--format-timezone-offset.el --- Tests for timezone offset formatting -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--format-timezone-offset.
+;; Converts offset seconds to human-readable "UTC±H:MM" strings.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--format-timezone-offset-normal-negative ()
+  "US Central (-6 hours) formats as UTC-6."
+  (should (equal "UTC-6" (calendar-sync--format-timezone-offset -21600))))
+
+(ert-deftest test-calendar-sync--format-timezone-offset-normal-positive ()
+  "India (+5:30) formats as UTC+5:30."
+  (should (equal "UTC+5:30" (calendar-sync--format-timezone-offset 19800))))
+
+(ert-deftest test-calendar-sync--format-timezone-offset-normal-utc ()
+  "Zero offset formats as UTC+0."
+  (should (equal "UTC+0" (calendar-sync--format-timezone-offset 0))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--format-timezone-offset-boundary-half-hour ()
+  "Newfoundland (-3:30) includes minutes."
+  (should (equal "UTC-3:30" (calendar-sync--format-timezone-offset -12600))))
+
+(ert-deftest test-calendar-sync--format-timezone-offset-boundary-large-offset ()
+  "UTC+14 (Line Islands) formats correctly."
+  (should (equal "UTC+14" (calendar-sync--format-timezone-offset 50400))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--format-timezone-offset-error-nil ()
+  "Nil offset returns 'unknown'."
+  (should (equal "unknown" (calendar-sync--format-timezone-offset nil))))
+
+(provide 'test-calendar-sync--format-timezone-offset)
+;;; test-calendar-sync--format-timezone-offset.el ends here