fix(icalendar): Correctly format nested objects (dates) in RRULE properties

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 {