Fix: Rounding error in get_data with tmin/tmax

mne/epochs.py

@@ -1595,6 +1595,34 @@ def _handle_empty(self, on_empty, meth):
             )
             _on_missing(on_empty, msg, error_klass=RuntimeError)
 
+    def _handle_tmin_tmax(self, tmin, tmax):
+        """Convert seconds to index into data."""
+        _validate_type(
+            tmin,
+            types=("numeric", None),
+            item_name="tmin",
+            type_name="int, float, None",
+        )
+        _validate_type(
+            tmax,
+            types=("numeric", None),
+            item_name="tmax",
+            type_name="int, float, None",
+        )
+
+        # handle tmin/tmax as start and stop indices into data array
+        n_times = self.times.size
+        start = 0 if tmin is None else self.time_as_index(tmin, use_rounding=True)[0]
+        stop = (
+            n_times if tmax is None else self.time_as_index(tmax, use_rounding=True)[0]
+        )
+
+        # truncate start/stop to the open interval [0, n_times]
+        start = min(max(0, start), n_times)
+        stop = min(max(0, stop), n_times)
+
+        return start, stop
+
     @verbose
     def _get_data(
         self,

mne/tests/test_epochs.py

@@ -4855,6 +4855,40 @@ def fun(data):
     assert_array_equal(out.get_data(non_picks), epochs.get_data(non_picks))
 
 
+def test_get_data_rounding():
+    """Test that get_data respects rounding for tmin/tmax (gh-13634)."""
+    # Data setup mirroring the issue report
+    data = np.linspace(-3.5, 1, 451).reshape((1, 1, 451))
+    info = create_info(["test"], 100.0, "eeg")
+    epochs = EpochsArray(data, info, tmin=-3.5, verbose=False)
+
+    t = 0.77
+
+    # compare crop() vs get_data()
+    # crop() uses proper rounding internally, get_data() should match it.
+    val_crop = epochs.copy().crop(tmin=t).get_data()[0, 0, 0]
+    val_get_data = epochs.get_data(tmin=t)[0, 0, 0]
+
+    assert_allclose(
+        val_get_data,
+        val_crop,
+        atol=1e-12,
+        err_msg="get_data(tmin) does not match crop(tmin)",
+    )
+
+    # verification on time consistency
+    # ensure we are getting the sample corresponding exactly to time 't'
+    idx = np.where(epochs.times == t)[0][0]
+    val_direct = epochs.get_data()[0, 0, idx]
+
+    assert_allclose(
+        val_get_data,
+        val_direct,
+        atol=1e-12,
+        err_msg="get_data(tmin) does not match direct indexing",
+    )
+
+
 def test_apply_function_epo_ch_access():
     """Test ch-access within apply function to epoch objects."""