fix: use SYS: prefix in pino-pretty translateTime to honour TZ env var

pino-pretty's translateTime: "HH:MM:ss" formats all timestamps in UTC,
ignoring the process TZ environment variable. Changing the prefix to
"SYS:HH:MM:ss" instructs pino-pretty to format timestamps in the local
system timezone, so operators running in non-UTC zones see correct
wall-clock times in both the console and the server.log file.

Fixes: #2879

server/src/__tests__/logger-tz.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, it, vi, beforeEach } from "vitest";
+
+/**
+ * Regression test for https://github.com/paperclipai/paperclip/issues/2879
+ *
+ * pino-pretty's `translateTime: "HH:MM:ss"` formats all timestamps in UTC
+ * regardless of the process's TZ env var. The `SYS:` prefix instructs
+ * pino-pretty to use the local system timezone, so operators in non-UTC
+ * zones see correct wall-clock times in their logs.
+ *
+ * We verify that:
+ * 1. The logger module initialises pino-pretty with "SYS:HH:MM:ss".
+ * 2. The SYS: approach actually produces timezone-aware output (via Node's
+ *    own Intl API, which mirrors what pino-pretty uses internally).
+ */
+
+const mockTransport = vi.hoisted(() => vi.fn(() => ({ write: vi.fn() })));
+const mockPino = vi.hoisted(() => {
+  const fn = vi.fn(() => ({
+    info: vi.fn(),
+    debug: vi.fn(),
+    error: vi.fn(),
+    warn: vi.fn(),
+    child: vi.fn(),
+  }));
+  (fn as any).transport = mockTransport;
+  return fn;
+});
+
+vi.mock("pino", () => ({
+  default: mockPino,
+}));
+vi.mock("pino-http", () => ({
+  pinoHttp: vi.fn(() => vi.fn()),
+}));
+vi.mock("../config-file.js", () => ({
+  readConfigFile: vi.fn(() => null),
+}));
+vi.mock("../home-paths.js", () => ({
+  resolveHomeAwarePath: vi.fn((p: string) => p),
+  resolveDefaultLogsDir: vi.fn(() => "/tmp/paperclip-test-logs"),
+}));
+
+describe("logger translateTime respects TZ environment variable", () => {
+  beforeEach(() => {
+    vi.resetModules();
+    mockTransport.mockClear();
+    mockPino.mockClear();
+  });
+
+  it("configures pino-pretty with SYS:HH:MM:ss so timestamps honour the TZ env var", async () => {
+    await import("../middleware/logger.js");
+
+    expect(mockTransport).toHaveBeenCalledOnce();
+    const { targets } = mockTransport.mock.calls[0][0] as { targets: Array<{ options: Record<string, unknown> }> };
+    for (const target of targets) {
+      expect(target.options.translateTime).toBe("SYS:HH:MM:ss");
+    }
+  });
+
+  it("SYS: behaviour: Node local-time formatting differs between UTC and UTC+8", () => {
+    // Demonstrates that using local time (what SYS: does) produces different
+    // output in different timezones — the property the fix relies on.
+    const EPOCH_MS = 946_684_800_000; // 2000-01-01 00:00:00 UTC
+
+    const fmtUtc = new Intl.DateTimeFormat("en-GB", {
+      timeZone: "UTC",
+      hour: "2-digit",
+      minute: "2-digit",
+      second: "2-digit",
+      hour12: false,
+    }).format(EPOCH_MS);
+
+    const fmtSgt = new Intl.DateTimeFormat("en-GB", {
+      timeZone: "Asia/Singapore",
+      hour: "2-digit",
+      minute: "2-digit",
+      second: "2-digit",
+      hour12: false,
+    }).format(EPOCH_MS);
+
+    // UTC midnight vs SGT 08:00 — must differ
+    expect(fmtUtc).toBe("00:00:00");
+    expect(fmtSgt).toBe("08:00:00");
+    expect(fmtUtc).not.toBe(fmtSgt);
+  });
+});

server/src/middleware/logger.ts

@@ -21,7 +21,7 @@ fs.mkdirSync(logDir, { recursive: true });
 const logFile = path.join(logDir, "server.log");
 
 const sharedOpts = {
-  translateTime: "HH:MM:ss",
+  translateTime: "SYS:HH:MM:ss",
   ignore: "pid,hostname",
   singleLine: true,
 };