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+;;; test-calendar-sync--add-days.el --- Tests for day arithmetic -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for calendar-sync--add-days. Uses noon internally for DST safety.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-calendar-sync)
+(require 'calendar-sync)
+
+;;; Normal Cases
+
+(ert-deftest test-calendar-sync--add-days-normal-forward ()
+  "Adding 7 days advances by one week."
+  (should (equal '(2026 3 22) (calendar-sync--add-days '(2026 3 15) 7))))
+
+(ert-deftest test-calendar-sync--add-days-normal-backward ()
+  "Negative days go backward."
+  (should (equal '(2026 3 8) (calendar-sync--add-days '(2026 3 15) -7))))
+
+;;; Boundary Cases
+
+(ert-deftest test-calendar-sync--add-days-boundary-month-rollover ()
+  "Adding days past end of month rolls into next month."
+  (should (equal '(2026 4 1) (calendar-sync--add-days '(2026 3 31) 1))))
+
+(ert-deftest test-calendar-sync--add-days-boundary-leap-year ()
+  "Feb 28 + 1 = Feb 29 in leap year 2028."
+  (should (equal '(2028 2 29) (calendar-sync--add-days '(2028 2 28) 1))))
+
+(ert-deftest test-calendar-sync--add-days-boundary-zero ()
+  "Adding zero days returns same date."
+  (should (equal '(2026 3 15) (calendar-sync--add-days '(2026 3 15) 0))))
+
+;;; Error Cases
+
+(ert-deftest test-calendar-sync--add-days-error-large-offset ()
+  "Adding 365 days crosses into next year."
+  (let ((result (calendar-sync--add-days '(2026 1 1) 365)))
+    (should (equal 2027 (nth 0 result)))))
+
+(provide 'test-calendar-sync--add-days)
+;;; test-calendar-sync--add-days.el ends here