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diff --git a/tests/test-calendar-sync--format-timestamp.el b/tests/test-calendar-sync--format-timestamp.el
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+++ b/tests/test-calendar-sync--format-timestamp.el
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+;;; test-calendar-sync--format-timestamp.el --- Tests for org timestamp formatting -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--format-timestamp.
+;; Converts parsed start/end times to org-mode timestamp strings.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--format-timestamp-normal-with-time ()
+  "Timed event produces '<YYYY-MM-DD Day HH:MM-HH:MM>'."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 3 15 14 0) '(2026 3 15 15 30))))
+    (should (string-match-p "^<2026-03-15 .* 14:00-15:30>$" result))))
+
+(ert-deftest test-calendar-sync--format-timestamp-normal-all-day ()
+  "All-day event (nil hours) produces '<YYYY-MM-DD Day>'."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 3 15 nil nil) '(2026 3 16 nil nil))))
+    (should (string-match-p "^<2026-03-15 .*>$" result))
+    (should-not (string-match-p "[0-9]:[0-9]" result))))
+
+(ert-deftest test-calendar-sync--format-timestamp-normal-includes-weekday ()
+  "Timestamp includes abbreviated weekday name."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 3 15 14 0) '(2026 3 15 15 0))))
+    ;; 2026-03-15 is a Sunday
+    (should (string-match-p "Sun" result))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--format-timestamp-boundary-midnight ()
+  "Midnight start and end are formatted as 00:00."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 1 1 0 0) '(2026 1 1 1 0))))
+    (should (string-match-p "00:00-01:00" result))))
+
+(ert-deftest test-calendar-sync--format-timestamp-boundary-nil-end ()
+  "Nil end with timed start still produces time range."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 3 15 14 0) nil)))
+    ;; No end time means no time range
+    (should (string-match-p "^<2026-03-15" result))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--format-timestamp-error-start-no-time-end-has-time ()
+  "All-day start with timed end produces date-only (no time range)."
+  (let ((result (calendar-sync--format-timestamp
+                 '(2026 3 15 nil nil) '(2026 3 15 15 0))))
+    ;; start-hour is nil so time-str should be nil
+    (should-not (string-match-p "[0-9][0-9]:[0-9][0-9]-" result))))
+
+(provide 'test-calendar-sync--format-timestamp)
+;;; test-calendar-sync--format-timestamp.el ends here