diff --git a/app/orm.py b/app/orm.py
index 50ff3ce..2082e2c 100644
--- a/app/orm.py
+++ b/app/orm.py
@@ -89,7 +89,7 @@ class Lease(Base):
             'lease_created': self.lease_created.isoformat(),
             'lease_expires': self.lease_expires.isoformat(),
             'lease_updated': self.lease_updated.isoformat(),
-            'lease_renewal': Lease.calculate_renewal(renewal_period, renewal_delta),
+            'lease_renewal': Lease.calculate_renewal(renewal_period, renewal_delta).isoformat(),
         }
 
     @staticmethod
@@ -158,7 +158,7 @@ class Lease(Base):
         return deletions
 
     @staticmethod
-    def calculate_renewal(renewal_period: float, delta: datetime.timedelta):
+    def calculate_renewal(renewal_period: float, delta: datetime.timedelta) -> datetime:
         """
         LEASE_RENEWAL_PERIOD=0.2  # 20%
         delta = datetime.timedelta(days=1)