diff --git a/src/anemoi/inference/runner.py b/src/anemoi/inference/runner.py
index 79cb75d..f8768e0 100644
--- a/src/anemoi/inference/runner.py
+++ b/src/anemoi/inference/runner.py
@@ -119,12 +119,33 @@ def run(
 
         LOGGER.info("Loading input: %d fields (lagged=%d)", len(input_fields), len(self.lagged))
 
-        input_fields_numpy = input_fields.to_numpy(dtype=np.float32)
+        if start_datetime is None:
+            start_datetime = input_fields.order_by(valid_datetime="ascending")[-1].metadata("valid_datetime")
+
+        num_fields_per_date = len(input_fields) // len(self.lagged)  # assumed
+
+        # Check valid_datetime of input data
+        # The subsequent reshape operation assumes that input_fields are chunkable by datetime
+        for i, lag in enumerate(self.lagged):
+            date = start_datetime + datetime.timedelta(hours=lag)
+            dates_found = set(
+                field.datetime() for field in input_fields[i * num_fields_per_date : (i + 1) * num_fields_per_date]
+            )
+            # All chunks must have the same datetime that must agree with the lag
+            if dates_found != {date}:
+                raise RuntimeError(
+                    "Inconsistent datetimes detected.\n"
+                    f"Datetimes in data: {', '.join(d.isoformat() for d in dates_found)}.\n"
+                    f"Expected datetime: {date.isoformat()} (for lag {lag})"
+                )
+
+        input_fields_numpy = input_fields.to_numpy(dtype=np.float32, reshape=False)
+
         print(input_fields_numpy.shape)
 
         input_fields_numpy = input_fields_numpy.reshape(
             len(self.lagged),
-            len(input_fields) // len(self.lagged),
+            num_fields_per_date,
             number_of_grid_points,
         )  # nlags, nparams, ngrid
 
@@ -223,10 +244,6 @@ def run(
             :, constant_data_from_retrieved_fields_mask
         ]
 
-        if start_datetime is None:
-            start_datetime_str = input_fields.order_by(valid_datetime="ascending")[-1].metadata("valid_datetime")
-            start_datetime = datetime.datetime.fromisoformat(start_datetime_str)
-
         constants = forcing_and_constants(
             source=input_fields[:1],
             param=self.checkpoint.computed_constants,