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diff --git a/tests/test-host-environment--detect-system-timezone.el b/tests/test-host-environment--detect-system-timezone.el
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+;;; test-host-environment--detect-system-timezone.el --- Tests for cj/detect-system-timezone -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for `cj/detect-system-timezone'.  The function tries four
+;; detection methods in priority order: file-content match against
+;; zoneinfo, the TZ env var, /etc/timezone, and the /etc/localtime
+;; symlink target.  Tests mock the first two methods to verify the
+;; priority chain without touching real system files.  Methods 3 and
+;; 4 (file I/O on /etc) are exercised end-to-end on the real host but
+;; not asserted strictly — those would be brittle across machines.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'host-environment)
+
+(ert-deftest test-host-environment-detect-tz-match-localtime-wins ()
+  "Normal: when match-localtime-to-zoneinfo returns a value, that wins."
+  (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+             (lambda () "America/Los_Angeles"))
+            ((symbol-function 'getenv)
+             (lambda (_) (error "TZ should not have been consulted"))))
+    (should (equal (cj/detect-system-timezone) "America/Los_Angeles"))))
+
+(ert-deftest test-host-environment-detect-tz-env-var-wins-when-match-nil ()
+  "Normal: with match-localtime nil, the TZ env var is used."
+  (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+             (lambda () nil))
+            ((symbol-function 'getenv)
+             (lambda (name) (when (string= name "TZ") "Europe/Berlin"))))
+    (should (equal (cj/detect-system-timezone) "Europe/Berlin"))))
+
+(ert-deftest test-host-environment-detect-tz-falls-through-to-etc-timezone ()
+  "Boundary: with match-localtime and TZ both nil, /etc/timezone is read.
+Uses a real temp file substituted via cl-letf on the file-existence and
+contents primitives."
+  (let ((fake-tz "Asia/Tokyo"))
+    (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+               (lambda () nil))
+              ((symbol-function 'getenv)
+               (lambda (_) nil))
+              ((symbol-function 'file-exists-p)
+               (lambda (path) (string= path "/etc/timezone")))
+              ((symbol-function 'insert-file-contents)
+               (lambda (path &rest _)
+                 (when (string= path "/etc/timezone")
+                   (insert fake-tz "\n")))))
+      (should (equal (cj/detect-system-timezone) fake-tz)))))
+
+(ert-deftest test-host-environment-detect-tz-trims-etc-timezone-whitespace ()
+  "Boundary: trailing whitespace in /etc/timezone is trimmed."
+  (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+             (lambda () nil))
+            ((symbol-function 'getenv)
+             (lambda (_) nil))
+            ((symbol-function 'file-exists-p)
+             (lambda (path) (string= path "/etc/timezone")))
+            ((symbol-function 'insert-file-contents)
+             (lambda (path &rest _)
+               (when (string= path "/etc/timezone")
+                 (insert "  America/Chicago\n\n")))))
+    (should (equal (cj/detect-system-timezone) "America/Chicago"))))
+
+(ert-deftest test-host-environment-detect-tz-returns-nil-when-all-fail ()
+  "Error: returns nil when every detection method fails."
+  (cl-letf (((symbol-function 'cj/match-localtime-to-zoneinfo)
+             (lambda () nil))
+            ((symbol-function 'getenv)
+             (lambda (_) nil))
+            ((symbol-function 'file-exists-p) (lambda (_) nil))
+            ((symbol-function 'file-symlink-p) (lambda (_) nil)))
+    (should-not (cj/detect-system-timezone))))
+
+(provide 'test-host-environment--detect-system-timezone)
+;;; test-host-environment--detect-system-timezone.el ends here