Make start_date optional for @continuous schedule

airflow-core/newsfragments/continuous-optional-start-date.improvement.rst

@@ -0,0 +1 @@
+The ``schedule="@continuous"`` parameter now works without requiring a ``start_date``, and any DAGs with this schedule will begin running immediately when unpaused.

airflow-core/src/airflow/timetables/simple.py

@@ -147,23 +147,21 @@ def next_dagrun_info(
         last_automated_data_interval: DataInterval | None,
         restriction: TimeRestriction,
     ) -> DagRunInfo | None:
-        if restriction.earliest is None:  # No start date, won't run.
-            return None
-
         current_time = timezone.coerce_datetime(timezone.utcnow())
+        start_date = restriction.earliest or current_time
 
         if last_automated_data_interval is not None:  # has already run once
             if last_automated_data_interval.end > current_time:  # start date is future
-                start = restriction.earliest
+                start = start_date
                 elapsed = last_automated_data_interval.end - last_automated_data_interval.start
 
                 end = start + elapsed.as_timedelta()
             else:
                 start = last_automated_data_interval.end
                 end = current_time
         else:  # first run
-            start = restriction.earliest
-            end = max(restriction.earliest, current_time)
+            start = start_date
+            end = max(start_date, current_time)
 
         if restriction.latest is not None and end > restriction.latest:
             return None

airflow-core/tests/unit/timetables/test_continuous_timetable.py

@@ -41,12 +41,28 @@ def timetable():
     return ContinuousTimetable()
 
 
-def test_no_runs_without_start_date(timetable):
+@time_machine.travel(DURING_DATE)
+def test_runs_without_start_date(timetable):
     next_info = timetable.next_dagrun_info(
         last_automated_data_interval=None,
         restriction=TimeRestriction(earliest=None, latest=None, catchup=False),
     )
-    assert next_info is None
+    assert next_info is not None
+    assert next_info.run_after == DURING_DATE
+    assert next_info.data_interval.start == DURING_DATE
+    assert next_info.data_interval.end == DURING_DATE
+
+
+@time_machine.travel(AFTER_DATE)
+def test_subsequent_runs_without_start_date(timetable):
+    next_info = timetable.next_dagrun_info(
+        last_automated_data_interval=DataInterval(DURING_DATE, DURING_DATE),
+        restriction=TimeRestriction(earliest=None, latest=None, catchup=False),
+    )
+    assert next_info is not None
+    assert next_info.run_after == AFTER_DATE
+    assert next_info.data_interval.start == DURING_DATE
+    assert next_info.data_interval.end == AFTER_DATE
 
 
 @time_machine.travel(DURING_DATE)