diff --git a/__tests__/claude-usage.test.js b/__tests__/claude-usage.test.js
index ae50546..ec08e58 100644
--- a/__tests__/claude-usage.test.js
+++ b/__tests__/claude-usage.test.js
@@ -736,6 +736,23 @@ describe("POST /claude-usage — payload", () => {
     expect(JSON.parse(fs.readFileSync(usagePath, "utf-8")).claudeCodeVersion).toBe("2.1.284");
   });
 
+  test("500 when the rename fails after the temp file is written, and the temp file is removed", async () => {
+    // The target is a non-empty directory: mkdir and the temp write succeed,
+    // renameSync fails. Only the cleanup in writeClaudeUsage() removes the
+    // temp file (#54: the mkdir case below never reaches it).
+    fs.mkdirSync(path.join(usagePath, "occupied"), { recursive: true });
+    const error = vi.spyOn(console, "error").mockImplementation(() => {});
+    try {
+      const res = await push();
+      expect(res.status).toBe(500);
+      expect(res.body.error).toBe("Internal error");
+    } finally {
+      error.mockRestore();
+    }
+    expect(fs.readdirSync(path.dirname(usagePath))).toEqual(["claude-usage.json"]);
+    expect(fs.readdirSync(usagePath)).toEqual(["occupied"]);
+  });
+
   test("500 when the snapshot cannot be written, and no temp file is left", async () => {
     // The parent of the file is a regular file: mkdir fails, nothing lands.
     const blocker = path.join(tmpDir, "blocker");
@@ -842,6 +859,11 @@ describe("GET /claude-usage", () => {
       ["another schemaVersion", JSON.stringify({ ...validUsage(), schemaVersion: 2, receivedAt: "2026-09-27T19:00:00.000Z" })],
       ["no receivedAt", JSON.stringify(validUsage())],
       ["an unusable receivedAt", JSON.stringify({ ...validUsage(), receivedAt: "never" })],
+      // new Date() reads all three; the server only ever stamps a full ISO
+      // timestamp with an offset, so anything else was not written by it (#54).
+      ["a receivedAt of \"1\"", JSON.stringify({ ...validUsage(), receivedAt: "1" })],
+      ["a date-only receivedAt", JSON.stringify({ ...validUsage(), receivedAt: "2026-10-07" })],
+      ["a receivedAt without an offset", JSON.stringify({ ...validUsage(), receivedAt: "2026-10-07T12:00:00" })],
     ])("on %s", async (_label, raw) => {
       writeStored(raw);
       const res = await read();
diff --git a/__tests__/hosts.test.js b/__tests__/hosts.test.js
index 8ed3212..a0bf8c8 100644
--- a/__tests__/hosts.test.js
+++ b/__tests__/hosts.test.js
@@ -869,6 +869,19 @@ describe("POST /hosts/<id> — what actually lands on disk", () => {
     expect(status).toBe(500);
     expect(body.error).toBe("Internal error");
   });
+
+  test("500 when the rename fails after the temp file is written, and the temp file is removed", async () => {
+    // The target is a non-empty directory: mkdir and the temp write succeed,
+    // renameSync fails. Only the cleanup in writeHostSnapshot() removes the
+    // temp file (#54: the case above fails at mkdir, before it exists).
+    fs.mkdirSync(path.join(hostsDir, "thinkpad.json", "occupied"), { recursive: true });
+
+    const { status, body } = await ingest("/hosts/thinkpad");
+    expect(status).toBe(500);
+    expect(body.error).toBe("Internal error");
+    expect(fs.readdirSync(hostsDir)).toEqual(["thinkpad.json"]);
+    expect(fs.readdirSync(path.join(hostsDir, "thinkpad.json"))).toEqual(["occupied"]);
+  });
 });
 
 describe("GET /hosts — freshness boundary", () => {
@@ -1010,6 +1023,14 @@ describe("GET /hosts — listing semantics", () => {
       "a receivedAt that is not a date",
       JSON.stringify({ id: "thinkpad", ...validSnapshot(), receivedAt: "yesterday" }),
     ],
+    // new Date() reads the next three; the server only ever stamps a full ISO
+    // timestamp with an offset, so anything else was not written by it (#54).
+    ['a receivedAt of "1"', JSON.stringify({ id: "thinkpad", ...validSnapshot(), receivedAt: "1" })],
+    ["a date-only receivedAt", JSON.stringify({ id: "thinkpad", ...validSnapshot(), receivedAt: "2026-10-07" })],
+    [
+      "a receivedAt without an offset",
+      JSON.stringify({ id: "thinkpad", ...validSnapshot(), receivedAt: "2026-10-07T12:00:00" }),
+    ],
     [
       "a receivedAt that is not a string",
       JSON.stringify({ id: "thinkpad", ...validSnapshot(), receivedAt: 1700000000000 }),
diff --git a/claude-usage.js b/claude-usage.js
index 7097c7c..bdeb207 100644
--- a/claude-usage.js
+++ b/claude-usage.js
@@ -68,6 +68,13 @@ function cleanTimestamp(value) {
   return value;
 }
 
+// Also used by index.js on the stored receivedAt of both snapshot files: the
+// server only ever stamps Date#toISOString(), and new Date() alone would
+// accept "1" or a bare date (#54).
+function isIsoTimestamp(value) {
+  return cleanTimestamp(value) !== undefined;
+}
+
 // The calendar check catches what the regex lets through (2026-02-31).
 function cleanDayDate(value) {
   if (typeof value !== "string" || !DAY_DATE_RE.test(value)) return undefined;
@@ -176,6 +183,7 @@ function sanitizeClaudeUsage(payload) {
 
 module.exports = {
   sanitizeClaudeUsage,
+  isIsoTimestamp,
   CLAUDE_USAGE_MAX_BODY_BYTES,
   SCHEMA_VERSION,
   MAX_DAYS,
diff --git a/index.js b/index.js
index 8fb7769..97c0715 100644
--- a/index.js
+++ b/index.js
@@ -13,7 +13,7 @@ const {
 const path = require("node:path");
 const { createHash, timingSafeEqual } = require("node:crypto");
 const { collectMetrics } = require("./metrics.js");
-const { sanitizeClaudeUsage, CLAUDE_USAGE_MAX_BODY_BYTES } = require("./claude-usage.js");
+const { sanitizeClaudeUsage, isIsoTimestamp, CLAUDE_USAGE_MAX_BODY_BYTES } = require("./claude-usage.js");
 
 const PORT = parseInt(process.env.PORT || "3001", 10);
 const TOKEN = process.env.HEALTH_TOKEN;
@@ -500,7 +500,7 @@ function readHostSnapshot(id) {
   }
 
   const receivedAt = typeof parsed.receivedAt === "string" ? parsed.receivedAt : null;
-  if (!receivedAt || Number.isNaN(new Date(receivedAt).getTime())) {
+  if (!isIsoTimestamp(receivedAt)) {
     console.error(`[hosts] snapshot for ${id} has no usable receivedAt`);
     return null;
   }
@@ -554,7 +554,7 @@ function readClaudeUsage() {
   }
 
   const receivedAt = typeof parsed.receivedAt === "string" ? parsed.receivedAt : null;
-  if (!receivedAt || Number.isNaN(new Date(receivedAt).getTime())) {
+  if (!isIsoTimestamp(receivedAt)) {
     console.error("[claude-usage] snapshot has no usable receivedAt");
     return null;
   }