diff --git a/coinmetrics/_data_collection.py b/coinmetrics/_data_collection.py
index 6394648..54f811d 100644
--- a/coinmetrics/_data_collection.py
+++ b/coinmetrics/_data_collection.py
@@ -640,7 +640,7 @@ def generate_ranges(
                 end_time = self.parse_date(
                     cast(datetime, self._url_params.get("end_time"))
                 ) if self._url_params.get(
-                    "end_time") else datetime.today()
+                    "end_time") else datetime.utcnow().replace(microsecond=0)
                 for start, end in generate_ranges(
                     start_time,
                     end_time,
diff --git a/test/test_parallel_datacollections.py b/test/test_parallel_datacollections.py
index 5e03edd..51e7f82 100644
--- a/test/test_parallel_datacollections.py
+++ b/test/test_parallel_datacollections.py
@@ -291,6 +291,19 @@ def test_height_increment() -> None:
         end_inclusive=False
     ).parallel(height_increment=10).to_dataframe()
     assert len(df_blocks) == 90
+    
+
+@pytest.mark.skipif(not cm_api_key_set, reason=REASON_TO_SKIP)
+def test_end_time_undefined() -> None:
+    start_time = datetime.datetime.utcnow().replace(microsecond=0, second=0) - timedelta(minutes=60)
+    df_metrics_1m = client.get_asset_metrics(
+        assets='btc', 
+        metrics='ReferenceRateUSD',
+        start_time=start_time,
+        frequency='1m'
+    ).parallel(time_increment=timedelta(minutes=1)).to_dataframe()
+    assert not df_metrics_1m.empty
+    assert df_metrics_1m.time.min().to_pydatetime().replace(tzinfo=None) == start_time
 
 
 if __name__ == '__main__':