diff --git a/src/components/NodeCard.tsx b/src/components/NodeCard.tsx
index 211fd9e..d0b4312 100644
--- a/src/components/NodeCard.tsx
+++ b/src/components/NodeCard.tsx
@@ -9,7 +9,7 @@ import { Badge } from "@/components/ui/badge"
 import { Card } from "@/components/ui/card"
 import { Meter } from "@/components/Meter"
 import type { Node } from "@/lib/api"
-import { bytes, daysUntil, FOREVER, osName, pair, percent, rate, uptime } from "@/lib/format"
+import { bytes, daysUntil, FOREVER, osName, pair, percent, rate, sinceSeen, uptime } from "@/lib/format"
 import { cn } from "@/lib/utils"
 
 // Emitted as files and fetched on first use, so a page carries only the flags its
@@ -69,7 +69,7 @@ function deployed(node: Node) {
  * durations, so the badge keeps its shape either way.
  */
 export function Status({ node }: { node: Node }) {
-  const down = node.last_seen ? Date.now() / 1000 - node.last_seen : 0
+  const down = sinceSeen(node)
   const label = node.online
     ? `在线 ${node.metrics ? uptime(node.metrics.uptime) : ""}`
     : deployed(node)
diff --git a/src/lib/api.ts b/src/lib/api.ts
index c305b85..43ded08 100644
--- a/src/lib/api.ts
+++ b/src/lib/api.ts
@@ -32,6 +32,8 @@ export type Node = {
   /** Set by the operator; empty is ungrouped. Absent from a hub predating groups. */
   group?: string
   last_seen: number
+  /** Seconds since `last_seen` on the hub's clock, null for a node never seen. Absent on older hubs. */
+  last_seen_ago?: number | null
   metrics: Metrics | null
   os: string
   kernel: string
diff --git a/src/lib/format.test.ts b/src/lib/format.test.ts
index 10f1b89..7cd2696 100644
--- a/src/lib/format.test.ts
+++ b/src/lib/format.test.ts
@@ -5,7 +5,7 @@
 // Nothing imports it, so the bundle never includes it.
 import {
   axisBytes, axisTop, bytes, cpuName, cycle, daysUntil, despike, money, osName, pair, percent, quarters,
-  RATE_FLOOR, rateAxis, tickClock, timeTicks, uptime, windows, withGaps,
+  RATE_FLOOR, rateAxis, sinceSeen, tickClock, timeTicks, uptime, windows, withGaps,
 } from "./format.ts"
 
 let failed = 0
@@ -136,6 +136,11 @@ eq(RATE_FLOOR, 1024, "底是 1 KB/s")
   eq(windows(1).map((w) => w.label), ["1 小时", "6 小时", "1 天"], "一天")
 }
 
+// sinceSeen: the hub's count wins, null included; only its absence falls back to the browser's.
+eq(sinceSeen({ last_seen: 1, last_seen_ago: 120 }), 120, "按 hub 的时钟")
+eq(sinceSeen({ last_seen: Date.now() / 1000 - 300, last_seen_ago: null }), 0, "hub 说从未上报，不改用浏览器计算")
+eq(Math.round(sinceSeen({ last_seen: Date.now() / 1000 - 300 })), 300, "旧 hub 由浏览器计算")
+
 // daysUntil: whole days, negative once past, null when there is no date.
 {
   const at = (days: number) => {
diff --git a/src/lib/format.ts b/src/lib/format.ts
index dc40680..91a5a66 100644
--- a/src/lib/format.ts
+++ b/src/lib/format.ts
@@ -365,3 +365,14 @@ function median(values: number[]): number {
   const half = sorted.length >> 1
   return sorted.length % 2 ? sorted[half] : (sorted[half - 1] + sorted[half]) / 2
 }
+
+/**
+ * Seconds since the node last reported, as the hub counts them: subtracted on
+ * the visitor's clock, a browser eight hours fast would show a node that
+ * dropped a minute ago as offline for eight hours. The browser's clock serves
+ * only a hub from before `last_seen_ago`.
+ */
+export function sinceSeen(node: { last_seen: number; last_seen_ago?: number | null }): number {
+  if (node.last_seen_ago !== undefined) return node.last_seen_ago ?? 0
+  return node.last_seen ? Date.now() / 1000 - node.last_seen : 0
+}