Warn if time monitor start time is after simulation run time

tests/test_components/test_simulation.py

@@ -1833,6 +1833,27 @@ def test_error_large_monitors():
             s.validate_pre_upload()
 
 
+@pytest.mark.parametrize("start, log_level", [(1e-12, None), (1, "WARNING")])
+def test_warn_time_monitor_outside_run_time(log_capture, start, log_level):
+    """Make sure we get a warning if the mode monitor grid is too large."""
+
+    sim = td.Simulation(
+        size=(2.0, 2.0, 2.0),
+        grid_spec=td.GridSpec.uniform(dl=0.1),
+        run_time=1e-12,
+        sources=[
+            td.ModeSource(
+                size=(0.1, 0.1, 0),
+                direction="+",
+                source_time=td.GaussianPulse(freq0=1e12, fwidth=0.1e12),
+            )
+        ],
+        monitors=[td.FieldTimeMonitor(size=(td.inf, 0, td.inf), start=start, name="test")],
+    )
+    with AssertLogLevel(log_capture, log_level_expected=log_level, contains_str="start time"):
+        sim.validate_pre_upload()
+
+
 def test_dt():
     """make sure dt is reduced when there is a medium with eps_inf < 1."""
     sim = td.Simulation(

tidy3d/components/simulation.py

@@ -29,7 +29,7 @@
 from .source import CustomCurrentSource, CustomSourceTime, ContinuousWave
 from .source import TFSF, Source, ModeSource
 from .monitor import MonitorType, Monitor, FreqMonitor, SurfaceIntegrationMonitor
-from .monitor import AbstractModeMonitor, FieldMonitor
+from .monitor import AbstractModeMonitor, FieldMonitor, TimeMonitor
 from .monitor import PermittivityMonitor, DiffractionMonitor, AbstractFieldProjectionMonitor
 from .monitor import FieldProjectionAngleMonitor, FieldProjectionKSpaceMonitor
 from .data.dataset import Dataset
@@ -1065,7 +1065,7 @@ def validate_pre_upload(self, source_required: bool = True) -> None:
         self._validate_modes_size()
         self._validate_datasets_not_none()
         self._validate_tfsf_structure_intersections()
-        # self._validate_run_time()
+        self._warn_time_monitors_outside_run_time()
         _ = self.volumetric_structures
         log.end_capture(self)
         if source_required and len(self.sources) == 0:
@@ -1282,6 +1282,21 @@ def _validate_tfsf_structure_intersections(self) -> None:
                             f" '{'xyz'[source.injection_axis]}'."
                         )
 
+    def _warn_time_monitors_outside_run_time(self) -> None:
+        """Warn if time monitors start after the simulation run_time.
+        TODO: (remove this comment later) this is done as a pre-upload validator in view of a
+        planned change to allow ``run_time`` to accept a ``RunTimeSpec`` which would automatically
+        determine a run time based on simulation details. Then, we would have to access the
+        dynamically computed run_time e.g. through a ``_run_time`` cached property.
+        """
+        with log as consolidated_logger:
+            for monitor in self.monitors:
+                if isinstance(monitor, TimeMonitor) and monitor.start > self.run_time:
+                    consolidated_logger.warning(
+                        f"Monitor {monitor.name} has a start time {monitor.start:1.2e}s exceeding"
+                        f"the simulation run time {self.run_time:1.2e}s. No data will be recorded."
+                    )
+
     """ Accounting """
 
     # candidate for removal in 3.0