diff --git a/docs/DOCKER.md b/docs/DOCKER.md
index 55e17f1f..59af9410 100644
--- a/docs/DOCKER.md
+++ b/docs/DOCKER.md
@@ -26,6 +26,10 @@ LOCATOR=FN20
 TZ=America/New_York
 ```
 
+> **`TZ` is optional** — if omitted, each visitor's browser timezone is used for
+> the local-time display. Setting it is still recommended so that server-side
+> timestamps (logs, cache TTLs, etc.) match your local time.
+
 Restart to apply:
 
 ```bash
diff --git a/server/routes/config-routes.js b/server/routes/config-routes.js
index e04eb7ab..ec9f88e3 100644
--- a/server/routes/config-routes.js
+++ b/server/routes/config-routes.js
@@ -173,8 +173,23 @@ module.exports = function (app, ctx) {
       // Whether config is incomplete (show setup wizard)
       configIncomplete: CONFIG.callsign === 'N0CALL' || !CONFIG.gridSquare,
 
-      // Server timezone (from TZ env var or system)
-      timezone: process.env.TZ || Intl.DateTimeFormat().resolvedOptions().timeZone || '',
+      // Server timezone (from TZ env var or system), validated.
+      // On minimal Linux containers without TZ set, Intl can return "Etc/Unknown"
+      // which browsers reject with RangeError. Validate before sending.
+      timezone: (() => {
+        const tz = process.env.TZ || Intl.DateTimeFormat().resolvedOptions().timeZone || '';
+        if (!tz) return '';
+        try {
+          new Intl.DateTimeFormat(undefined, { timeZone: tz });
+          return tz;
+        } catch (e) {
+          console.warn(
+            '[config] Invalid resolved timezone "%s" — falling back to empty (client will use browser TZ). Set TZ env var to silence.',
+            tz,
+          );
+          return '';
+        }
+      })(),
 
       // Feature availability
       features: {
diff --git a/src/hooks/app/useTimeState.js b/src/hooks/app/useTimeState.js
index 5b34e822..b6ff7af2 100644
--- a/src/hooks/app/useTimeState.js
+++ b/src/hooks/app/useTimeState.js
@@ -61,11 +61,24 @@ export default function useTimeState(configLocation, dxLocation, timezone) {
 
   const utcTime = currentTime.toISOString().substr(11, 8);
   const utcDate = currentTime.toISOString().substr(0, 10);
+  // Validate the timezone once per changed value, not on every render.
+  // new Intl.DateTimeFormat throws a RangeError for invalid values such as
+  // "Etc/Unknown" (returned by Node on minimal Linux containers with no TZ set).
+  const safeTimezone = useMemo(() => {
+    if (!timezone) return '';
+    try {
+      new Intl.DateTimeFormat(undefined, { timeZone: timezone });
+      return timezone;
+    } catch {
+      return '';
+    }
+  }, [timezone]);
+
   const localTimeOpts = { hour12: use12Hour };
   const localDateOpts = { weekday: 'short', month: 'short', day: 'numeric' };
-  if (timezone) {
-    localTimeOpts.timeZone = timezone;
-    localDateOpts.timeZone = timezone;
+  if (safeTimezone) {
+    localTimeOpts.timeZone = safeTimezone;
+    localDateOpts.timeZone = safeTimezone;
   }
   const localTime = currentTime.toLocaleTimeString('en-US', localTimeOpts);
   const localDate = currentTime.toLocaleDateString('en-US', localDateOpts);