diff --git a/icalendar.ts b/icalendar.ts
index bf5c638..302651a 100644
--- a/icalendar.ts
+++ b/icalendar.ts
@@ -29,6 +29,20 @@ const RRULE_KEY_MAP: Record<string, string> = {
   "freq": "FREQ", // Just in case
 };
 
+/**
+ * Formats an RRULE value for the string representation.
+ * Specifically handles Date objects and nested date objects from ts-ics.
+ */
+function formatRRuleValue(v: any): string {
+  if (v instanceof Date) {
+    return v.toISOString().replace(/[-:]/g, "").split(".")[0] + "Z";
+  }
+  if (typeof v === "object" && v !== null && v.date instanceof Date) {
+    return v.date.toISOString().replace(/[-:]/g, "").split(".")[0] + "Z";
+  }
+  return String(v);
+}
+
 // ============================================================================
 // Utility Functions
 // ============================================================================
@@ -186,7 +200,7 @@ export function expandRecurrences(icsEvent: any, windowDays = 365): any[] {
         cleanRule = Object.entries(rruleStr)
             .map(([k, v]) => {
                 const standardKey = RRULE_KEY_MAP[k.toLowerCase()] || k.toUpperCase();
-                return `${standardKey}=${v}`;
+                return `${standardKey}=${formatRRuleValue(v)}`;
             })
             .join(";");
     } else {
diff --git a/icalendar_test.ts b/icalendar_test.ts
index 1d5618f..3927868 100644
--- a/icalendar_test.ts
+++ b/icalendar_test.ts
@@ -207,7 +207,7 @@ Deno.test("expandRecurrences - object rrule (Reproduction of missing events)", (
 });
 
 
-Deno.test("expandRecurrences - object rrule with until (Reproduction)", () => {
+Deno.test("expandRecurrences - object rrule with until", () => {
   const now = new Date();
   const start = new Date(now.getTime() - 10 * 86400000); 
   const startStr = localDateString(start);
@@ -216,25 +216,12 @@ Deno.test("expandRecurrences - object rrule with until (Reproduction)", () => {
   const icsEvent = {
     summary: "Object RRULE UNTIL Event",
     start: startStr,
-    rrule: { frequency: "DAILY", until: { date: untilDate } } // Common ts-ics structure
+    rrule: { frequency: "DAILY", until: { date: untilDate } } 
   };
   
-  // Spy on console.error
-  let errorLogged = false;
-  const originalConsoleError = console.error;
-  console.error = (...args) => {
-    if (args[0].includes("Error expanding recurrence for Object RRULE UNTIL Event") && 
-        args[1].message.includes("Invalid UNTIL value")) {
-      errorLogged = true;
-    }
-  };
-
-  try {
-    expandRecurrences(icsEvent, 30);
-  } finally {
-    console.error = originalConsoleError;
-  }
-
-  assert(errorLogged, "Should have logged an error for invalid UNTIL object value");
+  const results = expandRecurrences(icsEvent, 30);
+  // Should now return multiple occurrences
+  assert(results.length > 1, `Expected > 1 occurrences, got ${results.length}`);
+  assertEquals(results[0].recurrent, true);
 });