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, 41 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--convert-utc-to-local.el b/tests/test-calendar-sync--convert-utc-to-local.el
new file mode 100644
index 00000000..1a5be4d3
--- /dev/null
+++ b/tests/test-calendar-sync--convert-utc-to-local.el
@@ -0,0 +1,41 @@
+;;; test-calendar-sync--convert-utc-to-local.el --- Tests for UTC to local conversion -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--convert-utc-to-local.
+;; Converts UTC datetime to local timezone. Results depend on system timezone.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--convert-utc-to-local-normal-returns-5-elements ()
+  "Conversion returns (year month day hour minute) list."
+  (let ((result (calendar-sync--convert-utc-to-local 2026 3 15 18 0 0)))
+    (should (= 5 (length result)))
+    (should (numberp (nth 0 result)))
+    (should (numberp (nth 3 result)))))
+
+(ert-deftest test-calendar-sync--convert-utc-to-local-normal-offset-applied ()
+  "Hour differs from UTC input (unless system is UTC)."
+  (let* ((tz-offset (car (current-time-zone)))
+         (result (calendar-sync--convert-utc-to-local 2026 3 15 12 0 0)))
+    (if (= tz-offset 0)
+        (should (= 12 (nth 3 result)))
+      (should-not (= 12 (nth 3 result))))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--convert-utc-to-local-boundary-date-change ()
+  "Late UTC time may shift to next day for western timezones."
+  (let* ((tz-offset (car (current-time-zone)))
+         (result (calendar-sync--convert-utc-to-local 2026 3 15 23 30 0)))
+    ;; For negative offsets, 23:30 UTC stays on same day
+    ;; For positive offsets > 30min, date rolls forward
+    (should (= 5 (length result)))))
+
+(provide 'test-calendar-sync--convert-utc-to-local)
+;;; test-calendar-sync--convert-utc-to-local.el ends here