diff --git a/src/logging/subsystem.ts b/src/logging/subsystem.ts
index 766f06b5678..653d8667432 100644
--- a/src/logging/subsystem.ts
+++ b/src/logging/subsystem.ts
@@ -184,7 +184,7 @@ function formatConsoleLine(opts: {
       return color.gray(`${h}:${m}:${s}`);
     }
     if (loggingState.consoleTimestampPrefix) {
-      return color.gray(now.toLocaleString("sv-SE", { timeZoneName: "short" }).replace(" ", "T"));
+      return color.gray(formatLocalIso(now));
     }
     return "";
   })();
diff --git a/src/logging/timestamp.test.ts b/src/logging/timestamp.test.ts
new file mode 100644
index 00000000000..7d7e54c104b
--- /dev/null
+++ b/src/logging/timestamp.test.ts
@@ -0,0 +1,85 @@
+import { describe, expect, it } from "vitest";
+import { formatLocalIso, localDateStr, localTimeStr, tzOffsetLabel } from "./timestamp.js";
+
+describe("formatLocalIso", () => {
+  it("formats a fixed date in local ISO-8601 with ms and tz offset", () => {
+    // 2026-06-15T10:30:45.123Z — use a fixed date
+    const d = new Date(2026, 5, 15, 10, 30, 45, 123); // local time
+    const result = formatLocalIso(d);
+
+    // Should start with local date/time
+    expect(result).toMatch(/^2026-06-15T10:30:45\.123[+-]\d{2}:\d{2}$/);
+  });
+
+  it("pads single-digit months/days/hours/minutes/seconds", () => {
+    const d = new Date(2026, 0, 5, 3, 7, 9, 4); // Jan 5, 03:07:09.004
+    const result = formatLocalIso(d);
+
+    expect(result).toMatch(/^2026-01-05T03:07:09\.004[+-]\d{2}:\d{2}$/);
+  });
+
+  it("uses current time when no argument is passed", () => {
+    const before = Date.now();
+    const result = formatLocalIso();
+    const after = Date.now();
+
+    // Parse the result back and verify it's within the time window
+    const parsed = new Date(result);
+    expect(parsed.getTime()).toBeGreaterThanOrEqual(before);
+    expect(parsed.getTime()).toBeLessThanOrEqual(after + 1);
+  });
+
+  it("produces a string parseable by new Date()", () => {
+    const original = new Date(2026, 7, 20, 14, 0, 0, 500);
+    const iso = formatLocalIso(original);
+    const reparsed = new Date(iso);
+
+    expect(reparsed.getTime()).toBe(original.getTime());
+  });
+
+  it("never ends with Z", () => {
+    const result = formatLocalIso(new Date());
+    expect(result).not.toMatch(/Z$/);
+    expect(result).toMatch(/[+-]\d{2}:\d{2}$/);
+  });
+});
+
+describe("localDateStr", () => {
+  it("returns YYYY-MM-DD from a local date", () => {
+    const d = new Date(2026, 11, 3, 23, 59, 59); // Dec 3
+    expect(localDateStr(d)).toBe("2026-12-03");
+  });
+
+  it("pads month and day", () => {
+    const d = new Date(2026, 0, 1); // Jan 1
+    expect(localDateStr(d)).toBe("2026-01-01");
+  });
+});
+
+describe("localTimeStr", () => {
+  it("returns HH:MM:SS from a local date", () => {
+    const d = new Date(2026, 0, 1, 14, 5, 9);
+    expect(localTimeStr(d)).toBe("14:05:09");
+  });
+
+  it("handles midnight", () => {
+    const d = new Date(2026, 0, 1, 0, 0, 0);
+    expect(localTimeStr(d)).toBe("00:00:00");
+  });
+});
+
+describe("tzOffsetLabel", () => {
+  it("returns a ±HH:MM string", () => {
+    const result = tzOffsetLabel(new Date());
+    expect(result).toMatch(/^[+-]\d{2}:\d{2}$/);
+  });
+
+  it("matches the offset in formatLocalIso", () => {
+    const d = new Date();
+    const iso = formatLocalIso(d);
+    const label = tzOffsetLabel(d);
+
+    // The last 6 chars of formatLocalIso should be the tz offset
+    expect(iso.slice(-6)).toBe(label);
+  });
+});