fix(core): keep Date values intact through log redaction

Fix `timestamp` and other `Date` fields being logged as `{}` in
structured log lines.

Mono-RevId: 3a9ec3f16fa49947100e0c6c8593c89aa4f1884b

Author: nicktrn

.changeset/structured-logger-date-fields.md

@@ -0,0 +1,5 @@
+---
+"@trigger.dev/core": patch
+---
+
+Fix `timestamp` and other `Date` fields being logged as `{}` in structured log lines.

packages/core/src/logger.ts

@@ -313,7 +313,7 @@ function filterKeys(obj: unknown, keys: Set<string>, depth = 0): any {
     return truncateString(obj);
   }
 
-  if (typeof obj !== "object" || obj === null) {
+  if (typeof obj !== "object" || obj === null || obj instanceof Date) {
     return obj;
   }
 

packages/core/test/logger.test.ts

@@ -186,6 +186,16 @@ describe("SimpleStructuredLogger redaction", () => {
     expect(line.harmless).toBe("keep me");
     expect(JSON.stringify(line)).not.toContain("should-not-appear");
   });
+
+  it("serialises Date values as ISO strings instead of walking them as objects", () => {
+    const logger = new SimpleStructuredLogger("test");
+    const startedAt = new Date("2026-10-20T12:00:00.000Z");
+
+    const line = captureLogLine(() => logger.info("run started", { startedAt }));
+
+    expect(line.timestamp).toMatch(/^\d{4}-\d{2}-\d{2}T/);
+    expect(line.startedAt).toBe("2026-10-20T12:00:00.000Z");
+  });
 });
 
 describe("redact()", () => {