diff --git a/src/aces/query.py b/src/aces/query.py
index 9f85876..d96f248 100644
--- a/src/aces/query.py
+++ b/src/aces/query.py
@@ -4,6 +4,7 @@
 """
 
 import logging
+from datetime import timedelta
 
 import polars as pl
 from bigtree import preorder_iter
@@ -11,6 +12,7 @@
 from .config import TaskExtractorConfig
 from .constraints import check_constraints, check_static_variables
 from .extract_subtree import extract_subtree
+from .types import TemporalWindowBounds
 from .utils import log_tree
 
 logger = logging.getLogger(__name__)
@@ -78,6 +80,49 @@ def query(cfg: TaskExtractorConfig, predicates_df: pl.DataFrame) -> pl.DataFrame
         True
         >>> "label" in result.columns
         True
+        >>> # Test backward window (start: end - 1 day, end: trigger): index_timestamp should be the
+        >>> # chronological end/start of the window, not the opposite end due to reversed summary timestamps.
+        >>> cfg_bwd = TaskExtractorConfig(
+        ...     predicates={},
+        ...     trigger=EventConfig("_ANY_EVENT"),
+        ...     windows={
+        ...         "input": WindowConfig("end - 1 day", "trigger", True, True, index_timestamp="end"),
+        ...     },
+        ... )
+        >>> predicates_bwd = pl.DataFrame({
+        ...     "subject_id": [1],
+        ...     "timestamp": [datetime(2010, 6, 20)],
+        ...     "_ANY_EVENT": [True],
+        ... })
+        >>> with caplog.at_level(logging.INFO):
+        ...     result_bwd = query(cfg_bwd, predicates_bwd)
+        >>> result_bwd.select("subject_id", "index_timestamp")
+        shape: (1, 2)
+        ┌────────────┬─────────────────────┐
+        │ subject_id ┆ index_timestamp     │
+        │ ---        ┆ ---                 │
+        │ i64        ┆ datetime[μs]        │
+        ╞════════════╪═════════════════════╡
+        │ 1          ┆ 2010-06-20 00:00:00 │
+        └────────────┴─────────────────────┘
+        >>> cfg_bwd_start = TaskExtractorConfig(
+        ...     predicates={},
+        ...     trigger=EventConfig("_ANY_EVENT"),
+        ...     windows={
+        ...         "input": WindowConfig("end - 1 day", "trigger", True, True, index_timestamp="start"),
+        ...     },
+        ... )
+        >>> with caplog.at_level(logging.INFO):
+        ...     result_bwd_start = query(cfg_bwd_start, predicates_bwd)
+        >>> result_bwd_start.select("subject_id", "index_timestamp")
+        shape: (1, 2)
+        ┌────────────┬─────────────────────┐
+        │ subject_id ┆ index_timestamp     │
+        │ ---        ┆ ---                 │
+        │ i64        ┆ datetime[μs]        │
+        ╞════════════╪═════════════════════╡
+        │ 1          ┆ 2010-06-19 00:00:00 │
+        └────────────┴─────────────────────┘
         >>> cfg = TaskExtractorConfig(
         ...     predicates={"A": PlainPredicateConfig("A", static=True)},
         ...     trigger=EventConfig("_ANY_EVENT"),
@@ -184,12 +229,23 @@ def query(cfg: TaskExtractorConfig, predicates_df: pl.DataFrame) -> pl.DataFrame
             f"Setting index timestamp as '{cfg.windows[cfg.index_timestamp_window].index_timestamp}' "
             f"of window '{cfg.index_timestamp_window}'..."
         )
-        index_timestamp_col = (
-            "end" if cfg.windows[cfg.index_timestamp_window].root_node == "start" else "start"
-        )
+        window_cfg = cfg.windows[cfg.index_timestamp_window]
+        index_timestamp_col = "end" if window_cfg.root_node == "start" else "start"
+        # When start_endpoint_expr is a TemporalWindowBounds with negative window_size
+        # (e.g., start: end - 24h), aggregate_temporal_window places the anchor time in
+        # timestamp_at_start and the earlier window start in timestamp_at_end (chronologically
+        # reversed). We must access the opposite field to get the correct chronological timestamp.
+        if (
+            window_cfg.root_node == "end"
+            and isinstance(window_cfg.start_endpoint_expr, TemporalWindowBounds)
+            and window_cfg.start_endpoint_expr.window_size < timedelta(0)
+        ):
+            timestamp_field = "start" if window_cfg.index_timestamp == "end" else "end"
+        else:
+            timestamp_field = window_cfg.index_timestamp
         result = result.with_columns(
             pl.col(f"{cfg.index_timestamp_window}.{index_timestamp_col}_summary")
-            .struct.field(f"timestamp_at_{cfg.windows[cfg.index_timestamp_window].index_timestamp}")
+            .struct.field(f"timestamp_at_{timestamp_field}")
             .alias("index_timestamp")
         )
         to_return_cols.insert(1, "index_timestamp")