diff --git a/facho/fe/form.py b/facho/fe/form.py
index d00ac60..629a825 100644
--- a/facho/fe/form.py
+++ b/facho/fe/form.py
@@ -5,7 +5,7 @@ import hashlib
 from functools import reduce
 import copy
 from dataclasses import dataclass
-from datetime import datetime
+from datetime import datetime, date
 from collections import defaultdict
 import decimal
 from decimal import Decimal
@@ -271,19 +271,31 @@ class PrePaidPayment:
 
 @dataclass
 class BillingReference:
-        def __init__(self, ident: str, uuid: str, date: str):
-            self.ident = ident
-            self.uuid = uuid
-            self.date = date
+    ident: str
+    uuid: str
+    date: date
 
 class CreditNoteDocumentReference(BillingReference):
-    pass
+    """
+    ident: Prefijo + Numero de la factura relacionada
+    uuid: CUFE de la factura electronica
+    date: fecha de emision de la factura relacionada
+    """
+
 
 class DebitNoteDocumentReference(BillingReference):
-    pass
+    """
+    ident: Prefijo + Numero de la factura relacionada
+    uuid: CUFE de la factura electronica
+    date: fecha de emision de la factura relacionada
+    """
 
 class InvoiceDocumentReference(BillingReference):
-    pass
+    """
+    ident: Prefijo + Numero de la nota credito relacionada
+    uuid: CUDE de la nota credito relacionada
+    date: fecha de emision de la nota credito relacionada
+    """
 
 @dataclass
 class InvoiceLine:
diff --git a/facho/fe/form_xml/invoice.py b/facho/fe/form_xml/invoice.py
index 63c4fd3..3170da0 100644
--- a/facho/fe/form_xml/invoice.py
+++ b/facho/fe/form_xml/invoice.py
@@ -400,7 +400,7 @@ class DIANInvoiceXML(fe.FeXML):
                           reference.uuid,
                           schemeName='CUFE-SHA384')
         fexml.set_element('./cac:BillingReference/cac:InvoiceDocumentReference/cbc:IssueDate',
-                          reference.date)
+                          reference.date.strftime("%Y-%m-%d"))
 
     def set_billing_reference(fexml, invoice):
         reference = invoice.invoice_billing_reference