diff --git a/env.py b/env.py
index b160dc2..0e412e4 100644
--- a/env.py
+++ b/env.py
@@ -70,6 +70,28 @@ def validate_date_format(env_var_name: str) -> str:
     return date_to_validate
 
 
+def validate_date_range(start_date: str, end_date: str) -> None:
+    """Validate that start_date is before end_date.
+
+    Does nothing if either date is not set.
+
+    Args:
+        start_date: The start date string in YYYY-MM-DD format.
+        end_date: The end date string in YYYY-MM-DD format.
+
+    Raises:
+        ValueError: If end_date is before or equal to start_date.
+    """
+    if not start_date or not end_date:
+        return
+
+    # YYYY-MM-DD format allows direct string comparison
+    if end_date <= start_date:
+        raise ValueError(
+            f"END_DATE ('{end_date}') must be after START_DATE ('{start_date}')"
+        )
+
+
 def get_env_vars(
     test: bool = False,
 ) -> tuple[
@@ -142,6 +164,7 @@ def get_env_vars(
 
     start_date = validate_date_format("START_DATE")
     end_date = validate_date_format("END_DATE")
+    validate_date_range(start_date, end_date)
 
     sponsor_info = get_bool_env_var("SPONSOR_INFO", False)
     link_to_profile = get_bool_env_var("LINK_TO_PROFILE", False)
diff --git a/test_env.py b/test_env.py
index 638e6e5..6059760 100644
--- a/test_env.py
+++ b/test_env.py
@@ -222,6 +222,65 @@ def test_get_env_vars_auth_with_github_app_installation_missing_inputs(self):
             "GH_APP_ID set and GH_APP_INSTALLATION_ID or GH_APP_PRIVATE_KEY variable not set",
         )
 
+    @patch.dict(
+        os.environ,
+        {
+            "ORGANIZATION": "org",
+            "REPOSITORY": "repo",
+            "GH_TOKEN": "token",
+            "START_DATE": "2025-01-01",
+            "END_DATE": "2024-01-01",
+        },
+        clear=True,
+    )
+    def test_get_env_vars_end_date_before_start_date(self):
+        """Test that an error is raised when END_DATE is before START_DATE"""
+        with self.assertRaises(ValueError) as cm:
+            env.get_env_vars()
+        the_exception = cm.exception
+        self.assertEqual(
+            str(the_exception),
+            "END_DATE ('2024-01-01') must be after START_DATE ('2025-01-01')",
+        )
+
+    @patch.dict(
+        os.environ,
+        {
+            "ORGANIZATION": "org",
+            "REPOSITORY": "repo",
+            "GH_TOKEN": "token",
+            "START_DATE": "2024-01-01",
+            "END_DATE": "2024-01-01",
+        },
+        clear=True,
+    )
+    def test_get_env_vars_equal_start_and_end_date(self):
+        """Test that an error is raised when START_DATE equals END_DATE"""
+        with self.assertRaises(ValueError) as cm:
+            env.get_env_vars()
+        the_exception = cm.exception
+        self.assertEqual(
+            str(the_exception),
+            "END_DATE ('2024-01-01') must be after START_DATE ('2024-01-01')",
+        )
+
+    @patch.dict(
+        os.environ,
+        {
+            "ORGANIZATION": "org",
+            "REPOSITORY": "repo",
+            "GH_TOKEN": "token",
+            "START_DATE": "2024-01-01",
+            "END_DATE": "2025-01-01",
+        },
+        clear=True,
+    )
+    def test_get_env_vars_valid_date_range(self):
+        """Test that valid date range (START_DATE before END_DATE) is accepted"""
+        result = env.get_env_vars()
+        self.assertEqual(result[8], "2024-01-01")  # start_date
+        self.assertEqual(result[9], "2025-01-01")  # end_date
+
 
 if __name__ == "__main__":
     unittest.main()