diff --git a/VERSION b/VERSION
index 67067fa..cde317c 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.3.137
\ No newline at end of file
+1.3.138
\ No newline at end of file
diff --git a/app/services/economic_service.py b/app/services/economic_service.py
index 07c097b..09ece98 100644
--- a/app/services/economic_service.py
+++ b/app/services/economic_service.py
@@ -451,17 +451,9 @@ class EconomicService:
         """
         try:
             # Calculate first day of the month 13 months ago (for yearly billed items)
+            from dateutil.relativedelta import relativedelta
             now = datetime.now()
-            # Go back 13 months
-            months_back = 13
-            if now.month > months_back:
-                start_month = now.month - months_back
-                start_year = now.year
-            else:
-                start_month = 12 - (months_back - now.month)
-                start_year = now.year - 1
-            
-            start_date = datetime(start_year, start_month, 1).strftime('%Y-%m-%d')
+            start_date = (now - relativedelta(months=13)).replace(day=1).strftime('%Y-%m-%d')
             logger.info(f"📅 Fetching invoices from {start_date} onwards (13 months for yearly billed items)")
             
             async with aiohttp.ClientSession() as session:
diff --git a/requirements.txt b/requirements.txt
index b85f036..3ccb257 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,6 +5,7 @@ pydantic==2.10.3
 pydantic-settings==2.6.1
 python-dotenv==1.0.1
 python-multipart==0.0.17
+python-dateutil==2.8.2
 jinja2==3.1.4
 aiohttp==3.10.10
 msal==1.31.1