Preserve milliseconds properly

Author: matt-aitken

apps/webapp/app/components/primitives/DateTime.tsx

@@ -133,7 +133,9 @@ export function formatDateTimeISO(date: Date, timeZone: string): string {
   // Format: YYYY-MM-DDThh:mm:ss.sss±hh:mm
   return (
     `${dateParts.year}-${dateParts.month}-${dateParts.day}T` +
-    `${dateParts.hour}:${dateParts.minute}:${dateParts.second}.000${offset}`
+    `${dateParts.hour}:${dateParts.minute}:${dateParts.second}.${String(
+      date.getMilliseconds()
+    ).padStart(3, "0")}${offset}`
   );
 }
 

apps/webapp/test/components/DateTime.test.ts

@@ -45,4 +45,10 @@ describe("formatDateTimeISO", () => {
     const result = formatDateTimeISO(date, "UTC");
     expect(result).toBe("2025-04-29T14:01:19.123Z");
   });
+
+  it("should preserve milliseconds, not UTC", () => {
+    const date = new Date("2025-04-29T14:01:19.123Z");
+    const result = formatDateTimeISO(date, "Europe/London");
+    expect(result).toBe("2025-04-29T15:01:19.123+01:00");
+  });
 });