diff --git a/python/activator/middleware_interface.py b/python/activator/middleware_interface.py
index 5665714e..443df4c2 100644
--- a/python/activator/middleware_interface.py
+++ b/python/activator/middleware_interface.py
@@ -123,6 +123,10 @@ def make_local_repo(local_storage: str, central_butler: Butler, instrument: str)
     return repo_dir
 
 
+# Time zone used to define exposures' day_obs value.
+_DAY_OBS_TZ = datetime.timezone(datetime.timedelta(hours=-12), name="day_obs")
+
+
 class MiddlewareInterface:
     """Interface layer between the Butler middleware and the prompt processing
     data handling system, to handle processing individual images.
@@ -572,8 +576,8 @@ def _get_init_output_run(self,
         Parameters
         ----------
         date : `datetime.date`, optional
-            Date of the processing run (not observation!), defaults to the UTC
-            date this method was called.
+            Date of the processing run (not observation!), defaults to the
+            day_obs this method was called.
 
         Returns
         -------
@@ -581,7 +585,7 @@ def _get_init_output_run(self,
             The run in which to place pipeline init-outputs.
         """
         if date is None:
-            date = datetime.datetime.now(datetime.timezone.utc)
+            date = datetime.datetime.now(_DAY_OBS_TZ)
         # Current executor requires that init-outputs be in the same run as
         # outputs. This can be changed once DM-36162 is done.
         return self._get_output_run(date)
@@ -594,8 +598,8 @@ def _get_output_run(self,
         Parameters
         ----------
         date : `datetime.date`, optional
-            Date of the processing run (not observation!), defaults to the UTC
-            date this method was called.
+            Date of the processing run (not observation!), defaults to the
+            day_obs this method was called.
 
         Returns
         -------
@@ -603,7 +607,7 @@ def _get_output_run(self,
             The run in which to place processing outputs.
         """
         if date is None:
-            date = datetime.datetime.now(datetime.timezone.utc)
+            date = datetime.datetime.now(_DAY_OBS_TZ)
         pipeline_name, _ = os.path.splitext(os.path.basename(self._get_pipeline_file()))
         # Order optimized for S3 bucket -- filter out as many files as soon as possible.
         return self.instrument.makeCollectionName(
diff --git a/tests/test_middleware_interface.py b/tests/test_middleware_interface.py
index 679b6b8c..fea20bd8 100644
--- a/tests/test_middleware_interface.py
+++ b/tests/test_middleware_interface.py
@@ -410,6 +410,25 @@ def test_get_output_run(self):
                              "/ApPipe/prompt-proto-service-042"
                              )
 
+    def test_get_output_run_default(self):
+        # Workaround for mocking builtin class; see
+        # https://williambert.online/2011/07/how-to-unit-testing-in-django-with-mocking-and-patching/
+        class MockDatetime(datetime.datetime):
+            @classmethod
+            def now(cls, tz=None):
+                # This time will be the same day in CLT/CLST, but the previous day in day_obs.
+                utc = datetime.datetime(2023, 3, 15, 5, 42, 3, tzinfo=datetime.timezone.utc)
+                if tz:
+                    return utc.astimezone(tz)
+                else:
+                    return utc.replace(tzinfo=None)
+
+        with unittest.mock.patch("datetime.datetime", MockDatetime):
+            out_run = self.interface._get_output_run()
+            self.assertIn("output-2023-03-14", out_run)
+            init_run = self.interface._get_init_output_run()
+            self.assertIn("output-2023-03-14", init_run)
+
     def _assert_in_collection(self, butler, collection, dataset_type, data_id):
         # Pass iff any dataset matches the query, no need to check them all.
         for dataset in butler.registry.queryDatasets(dataset_type, collections=collection, dataId=data_id):