diff --git a/RELEASE_NOTES.rst b/RELEASE_NOTES.rst
index 66edcf9c90e90..33cae19c5cea2 100644
--- a/RELEASE_NOTES.rst
+++ b/RELEASE_NOTES.rst
@@ -1244,6 +1244,13 @@ DaskExecutor - Dask Worker Resources and queues
 
 If dask workers are not started with complementary resources to match the specified queues, it will now result in an ``AirflowException``\ , whereas before it would have just ignored the ``queue`` argument.
 
+Logical date of a DAG run triggered from the web UI now have its sub-second component set to zero
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+Due to a change in how the logical date (``execution_date``) is generated for a manual DAG run, a manual DAG run’s logical date may not match its time-of-trigger, but have its sub-second part zero-ed out. For example, a DAG run triggered on ``2021-10-11T12:34:56.78901`` would have its logical date set to ``2021-10-11T12:34:56.00000``.
+
+This may affect some logic that expects on this quirk to detect whether a run is triggered manually or not. Note that ``dag_run.run_type`` is a more authoritative value for this purpose. Also, if you need this distinction between automated and manually-triggered rus for “next execution date” calculation, please also consider using the new data interval variables instead, which provide a more consistent behavior between the two run types.
+
 New Features
 ^^^^^^^^^^^^