diff --git a/piker/data/_pathops.py b/piker/data/_pathops.py
index d8c15511..48a11f40 100644
--- a/piker/data/_pathops.py
+++ b/piker/data/_pathops.py
@@ -295,7 +295,7 @@ def slice_from_time(
     arr: np.ndarray,
     start_t: float,
     stop_t: float,
-    step: int | None = None,
+    step: float,  # sampler period step-diff
 
 ) -> slice:
     '''
@@ -324,12 +324,6 @@ def slice_from_time(
     # end of the input array.
     read_i_max = arr.shape[0]
 
-    # TODO: require this is always passed in?
-    if step is None:
-        step = round(t_last - times[-2])
-        if step == 0:
-            step = 1
-
     # compute (presumed) uniform-time-step index offsets
     i_start_t = floor(start_t)
     read_i_start = floor(((i_start_t - t_first) // step)) - 1
@@ -412,7 +406,7 @@ def slice_from_time(
             times[read_i_start:],
             # times,
             i_stop_t,
-            side='left',
+            side='right',
         )
 
         if (