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+;;; test-calendar-sync--nth-weekday-of-month.el --- Tests for calendar-sync--nth-weekday-of-month  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the nth-weekday-of-month helper backing monthly/yearly BYDAY
+;; expansion.  Fixed dates are safe here: the function is pure calendar
+;; arithmetic with no relation to the current time.
+
+;;; Code:
+
+(require 'ert)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-normal-second-wednesday ()
+  "Normal: 2nd Wednesday of Jan 2026 is the 14th."
+  ;; Jan 2026: Jan 1 is a Thursday; Wednesdays fall on 7, 14, 21, 28.
+  (should (= (calendar-sync--nth-weekday-of-month 2026 1 3 2) 14)))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-normal-first-monday ()
+  "Normal: 1st Monday of Feb 2026 is the 2nd."
+  ;; Feb 2026: Feb 1 is a Sunday; Mondays fall on 2, 9, 16, 23.
+  (should (= (calendar-sync--nth-weekday-of-month 2026 2 1 1) 2)))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-normal-last-tuesday ()
+  "Normal: last Tuesday of Mar 2026 is the 31st (negative ordinal)."
+  ;; Mar 2026: Tuesdays fall on 3, 10, 17, 24, 31.
+  (should (= (calendar-sync--nth-weekday-of-month 2026 3 2 -1) 31)))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-normal-second-to-last-friday ()
+  "Normal: -2 ordinal picks the second-to-last Friday."
+  ;; May 2026: Fridays fall on 1, 8, 15, 22, 29.
+  (should (= (calendar-sync--nth-weekday-of-month 2026 5 5 -2) 22)))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-boundary-fifth-occurrence-exists ()
+  "Boundary: 5th Friday exists in May 2026."
+  (should (= (calendar-sync--nth-weekday-of-month 2026 5 5 5) 29)))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-boundary-fifth-occurrence-missing ()
+  "Boundary: 5th Wednesday of Feb 2026 does not exist -- returns nil."
+  ;; Feb 2026 has four Wednesdays (4, 11, 18, 25).
+  (should (null (calendar-sync--nth-weekday-of-month 2026 2 3 5))))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-boundary-first-day-is-target ()
+  "Boundary: the 1st of the month itself is the 1st occurrence."
+  ;; Apr 2026: Apr 1 is a Wednesday.
+  (should (= (calendar-sync--nth-weekday-of-month 2026 4 3 1) 1)))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-boundary-leap-february ()
+  "Boundary: leap-year February (2028) handled -- last Tuesday is the 29th."
+  ;; Feb 2028: Feb 29 exists and is a Tuesday.
+  (should (= (calendar-sync--nth-weekday-of-month 2028 2 2 -1) 29)))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-error-zero-ordinal-nil ()
+  "Error: ordinal 0 is meaningless -- returns nil."
+  (should (null (calendar-sync--nth-weekday-of-month 2026 1 3 0))))
+
+(ert-deftest test-calendar-sync--nth-weekday-of-month-error-out-of-range-negative-nil ()
+  "Error: -6th occurrence never exists in a month -- returns nil."
+  (should (null (calendar-sync--nth-weekday-of-month 2026 1 3 -6))))
+
+(provide 'test-calendar-sync--nth-weekday-of-month)
+;;; test-calendar-sync--nth-weekday-of-month.el ends here