diff --git a/src/aces/extract_subtree.py b/src/aces/extract_subtree.py
index 7663436d..d1dcb8a1 100644
--- a/src/aces/extract_subtree.py
+++ b/src/aces/extract_subtree.py
@@ -54,6 +54,7 @@ def extract_subtree(
     Examples:
         >>> from bigtree import Node
         >>> from datetime import datetime
+        >>> from .types import ToEventWindowBounds, TemporalWindowBounds
         >>> # We'll use an example for in-hospital mortality prediction. Our root event of the tree will be
         >>> # an admission event.
         >>> root = Node("admission")
@@ -63,7 +64,7 @@ def extract_subtree(
         >>> # Node 1 will represent our gap window. We say that in the 24 hours after the admission, there
         >>> # should be no discharges, deaths, or covid events.
         >>> gap_node = Node("gap") # This sets the node's name.
-        >>> gap_node.endpoint_expr = (True, timedelta(days=2), True)
+        >>> gap_node.endpoint_expr = TemporalWindowBounds(True, timedelta(days=2), True)
         >>> gap_node.constraints = {
         ...     "is_discharge": (None, 0), "is_death": (None, 0), "is_covid_dx": (None, 0)
         ... }
@@ -71,7 +72,7 @@ def extract_subtree(
         >>> # Node 2 will start our target window and span until the next discharge or death event.
         >>> # There should be no covid events.
         >>> target_node = Node("target") # This sets the node's name.
-        >>> target_node.endpoint_expr = (True, "is_discharge", True)
+        >>> target_node.endpoint_expr = ToEventWindowBounds(True, "is_discharge", True)
         >>> target_node.constraints = {"is_covid_dx": (None, 0)}
         >>> target_node.parent = gap_node
         >>> #
@@ -79,11 +80,11 @@ def extract_subtree(
         >>> # Finally, for our second branch, we will impose no constraints but track the input time range,
         >>> # which will span from the beginning of the record to 24 hours after admission.
         >>> input_end_node = Node("input_end")
-        >>> input_end_node.endpoint_expr = (True, timedelta(days=1), True)
+        >>> input_end_node.endpoint_expr = TemporalWindowBounds(True, timedelta(days=1), True)
         >>> input_end_node.constraints = {}
         >>> input_end_node.parent = root
         >>> input_start_node = Node("input_start")
-        >>> input_start_node.endpoint_expr = (True, "-_RECORD_START", True)
+        >>> input_start_node.endpoint_expr = ToEventWindowBounds(True, "-_RECORD_START", True)
         >>> input_start_node.constraints = {}
         >>> input_start_node.parent = root
         >>> #
@@ -93,11 +94,11 @@ def extract_subtree(
         >>> # This will be expressed through two windows, one spanning back a year, and the other looking
         >>> # prior to that year.
         >>> pre_node_1yr = Node("pre_node_1yr")
-        >>> pre_node_1yr.endpoint_expr = (False, timedelta(days=-365), False)
+        >>> pre_node_1yr.endpoint_expr = TemporalWindowBounds(False, timedelta(days=-365), False)
         >>> pre_node_1yr.constraints = {}
         >>> pre_node_1yr.parent = root
         >>> pre_node_total = Node("pre_node_total")
-        >>> pre_node_total.endpoint_expr = (False, "-_RECORD_START", False)
+        >>> pre_node_total.endpoint_expr = ToEventWindowBounds(False, "-_RECORD_START", False)
         >>> pre_node_total.constraints = {"*": (1, None)}
         >>> pre_node_total.parent = pre_node_1yr
         >>> #
@@ -272,11 +273,16 @@ def extract_subtree(
                 # In an event bound case, the child root will be a proper extant event, so it will be the
                 # anchor as well, and thus the child root offset should be zero.
                 child_root_offset = timedelta(days=0)
+                if endpoint_expr.end_event.startswith("-"):
+                    child_anchor_time = "timestamp_at_start"
+                else:
+                    child_anchor_time = "timestamp_at_end"
+
                 window_summary_df = (
                     aggregate_event_bound_window(predicates_df, endpoint_expr)
                     .with_columns(
                         pl.col("timestamp").alias("subtree_anchor_timestamp"),
-                        pl.col("timestamp_at_end").alias("child_anchor_timestamp"),
+                        pl.col(child_anchor_time).alias("child_anchor_timestamp"),
                     )
                     .drop("timestamp")
                 )