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, 35 insertions, 0 deletions

diff --git a/tests/test-calendar-sync--date-weekday.el b/tests/test-calendar-sync--date-weekday.el
new file mode 100644
index 00000000..61a499bd
--- /dev/null
+++ b/tests/test-calendar-sync--date-weekday.el
@@ -0,0 +1,35 @@
+;;; test-calendar-sync--date-weekday.el --- Tests for date weekday calculation -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--date-weekday. Returns 1 (Mon) through 7 (Sun).
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--date-weekday-normal-known-date ()
+  "2026-03-15 is a Sunday (7)."
+  (should (= 7 (calendar-sync--date-weekday '(2026 3 15)))))
+
+(ert-deftest test-calendar-sync--date-weekday-normal-monday ()
+  "2026-03-16 is a Monday (1)."
+  (should (= 1 (calendar-sync--date-weekday '(2026 3 16)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--date-weekday-boundary-sunday-is-7 ()
+  "Sunday returns 7, not 0 (Emacs decode-time returns 0 for Sunday)."
+  ;; 2026-01-04 is a Sunday
+  (should (= 7 (calendar-sync--date-weekday '(2026 1 4)))))
+
+(ert-deftest test-calendar-sync--date-weekday-boundary-saturday ()
+  "Saturday returns 6."
+  ;; 2026-01-03 is a Saturday
+  (should (= 6 (calendar-sync--date-weekday '(2026 1 3)))))
+
+(provide 'test-calendar-sync--date-weekday)
+;;; test-calendar-sync--date-weekday.el ends here