diff --git a/src/weathergen/datasets/data_reader_base.py b/src/weathergen/datasets/data_reader_base.py
index 66c36c39d..e9ddcb255 100644
--- a/src/weathergen/datasets/data_reader_base.py
+++ b/src/weathergen/datasets/data_reader_base.py
@@ -188,6 +188,9 @@ def window(self, idx: TIndex) -> DTRange:
 
         return DTRange(t_start_win, t_end_win)
 
+    def get_n_steps(self, forecast_step: int) -> int:
+        return (int(self.t_window_len) * forecast_step) // int(self.t_window_step)
+
 
 @dataclass
 class ReaderData:
diff --git a/src/weathergen/datasets/multi_stream_data_sampler.py b/src/weathergen/datasets/multi_stream_data_sampler.py
index 83adb8128..9a6c9cd84 100644
--- a/src/weathergen/datasets/multi_stream_data_sampler.py
+++ b/src/weathergen/datasets/multi_stream_data_sampler.py
@@ -243,7 +243,7 @@ def reset(self):
         # value in worker_workset()
         self.rng = np.random.default_rng(self.data_loader_rng_seed)
 
-        fsm = (
+        fsm: int = (
             self.forecast_steps[min(self.epoch, len(self.forecast_steps) - 1)]
             if self.forecast_policy != "random"
             else self.forecast_steps.max()
@@ -255,7 +255,7 @@ def reset(self):
         index_range = self.time_window_handler.get_index_range()
         idx_end = index_range.end
         # native length of datasets, independent of epoch length that has potentially been specified
-        forecast_len = (self.len_hrs * (fsm + 1)) // self.step_hrs
+        forecast_len = self.time_window_handler.get_n_steps(fsm + 1)
         idx_end -= forecast_len + self.forecast_offset
         assert idx_end > 0, "dataset size too small for forecast range"
         self.perms = np.arange(index_range.start, idx_end)
@@ -309,16 +309,19 @@ def __iter__(self):
         # bidx is used to count the #batches that have been emitted
         # idx_raw is used to index into the dataset; the decoupling is needed
         # since there are empty batches
-        idx_raw = iter_start
-        for i, _bidx in enumerate(range(iter_start, iter_end, self.batch_size)):
-            # forecast_dt needs to be constant per batch (amortized through data parallel training)
-            forecast_dt = self.perms_forecast_dt[i]
-
+        idx_raw = iter_start  # start step index
+        assert (iter_end - iter_start) // self.batch_size == len(self.perms_forecast_dt)
+        # forecast_dt needs to be constant per batch (amortized through data parallel training)
+        for forecast_dt in self.perms_forecast_dt:  # bidx loop
             # use while loop due to the scattered nature of the data in time and to
             # ensure batches are not empty
             batch = []
             while len(batch) < self.batch_size:
-                idx: TIndex = self.perms[idx_raw % self.perms.shape[0]]
+                # TODO: identity? len(self.perms) should be most likely longer then
+                # idx_raw since it contains the all dataset steps (- small adjustment)
+                # whereas iter_end-iter_start should be smaller since it is only a subset
+                perm_idx = idx_raw % len(self.perms)
+                idx: TIndex = self.perms[perm_idx]
                 idx_raw += 1
 
                 time_win1 = self.time_window_handler.window(idx)
@@ -373,12 +376,10 @@ def __iter__(self):
                         for fstep in range(
                             self.forecast_offset, self.forecast_offset + forecast_dt + 1
                         ):
-                            step_forecast_dt = (
-                                idx + (self.forecast_delta_hrs * fstep) // self.step_hrs
-                            )
-                            time_win2 = self.time_window_handler.window(step_forecast_dt)
+                            forecast_idx = idx + self.time_window_handler.get_n_steps(fstep)
+                            time_win2 = self.time_window_handler.window(forecast_idx)
 
-                            rdata = ds.get_target(step_forecast_dt)
+                            rdata = ds.get_target(forecast_idx)
 
                             sample_is_empty = rdata.is_empty()
                             if sample_is_empty: