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diff --git a/tests/test-calendar-sync--event-start-time.el b/tests/test-calendar-sync--event-start-time.el
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+;;; test-calendar-sync--event-start-time.el --- Tests for event start time extraction -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--event-start-time. Extracts comparable time value from event plist.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--event-start-time-normal-timed ()
+  "Timed event returns non-zero time value."
+  (let ((event (list :start '(2026 3 15 14 30) :summary "Test")))
+    (should (not (equal 0 (calendar-sync--event-start-time event))))))
+
+(ert-deftest test-calendar-sync--event-start-time-normal-ordering ()
+  "Earlier event has smaller time value than later event."
+  (let ((early (list :start '(2026 3 15 9 0)))
+        (late (list :start '(2026 3 15 17 0))))
+    (should (time-less-p (calendar-sync--event-start-time early)
+                         (calendar-sync--event-start-time late)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--event-start-time-boundary-all-day ()
+  "All-day event (nil hour/minute) uses 0 for time components."
+  (let ((event (list :start '(2026 3 15 nil nil))))
+    (should (not (equal 0 (calendar-sync--event-start-time event))))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--event-start-time-error-no-start ()
+  "Event without :start returns 0."
+  (let ((event (list :summary "No start")))
+    (should (equal 0 (calendar-sync--event-start-time event)))))
+
+(provide 'test-calendar-sync--event-start-time)
+;;; test-calendar-sync--event-start-time.el ends here