diff --git a/litellm/litellm_core_utils/llm_cost_calc/utils.py b/litellm/litellm_core_utils/llm_cost_calc/utils.py
index 2f0e2a61086..27ef9176c13 100644
--- a/litellm/litellm_core_utils/llm_cost_calc/utils.py
+++ b/litellm/litellm_core_utils/llm_cost_calc/utils.py
@@ -4,7 +4,7 @@
 import re
 from collections.abc import Mapping, Sequence
 from dataclasses import dataclass
-from datetime import datetime, timezone, tzinfo
+from datetime import date, datetime, timezone, tzinfo
 from types import MappingProxyType
 from typing import Final, Literal, TypedDict, cast
 from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
@@ -492,10 +492,23 @@ def _as_window_strings(value: object) -> tuple[str, ...]:
     return ()
 
 
+def _is_iso_date(value: object) -> bool:
+    if not isinstance(value, str) or len(value) != 10:
+        return False
+    try:
+        date.fromisoformat(value)
+    except ValueError:
+        return False
+    return True
+
+
 def _as_date_strings(value: object) -> tuple[str, ...]:
     if isinstance(value, str) or not isinstance(value, Sequence):
         return ()
-    return tuple(entry for entry in value if isinstance(entry, str))
+    entries: Final = tuple(value)
+    if not entries or not all(_is_iso_date(entry) for entry in entries):
+        return ()
+    return entries
 
 
 def _is_off_peak(off_peak: Mapping[str, object], current_time: datetime | None = None) -> bool:
diff --git a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
index 0f61bbdf674..4818f378b1d 100644
--- a/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
+++ b/tests/test_litellm/litellm_core_utils/llm_cost_calc/test_llm_cost_calc_utils.py
@@ -724,6 +724,22 @@ def test_is_off_peak_ignores_malformed_override_dates():
         )
 
 
+def test_is_off_peak_disables_rules_with_partially_malformed_override_dates():
+    """One bad entry disables the whole override_dates list: a rule whose dates do not all
+    parse never applies on any date, so the flat hours decide alone."""
+    from datetime import datetime, timezone
+
+    peak_instant = datetime(2026, 3, 4, 12, 0, tzinfo=timezone.utc)
+    for bad in (["2026-03-04", 20260304], ["2026-03-04", "2026-3-5"]):
+        block = {
+            "hours_utc": "00:30-01:00",
+            "windows": [{"hours_utc": "00:00-00:00", "override_dates": bad}],
+        }
+        assert _is_off_peak(block, peak_instant) is False, (
+            f"override_dates={bad!r} must disable its rule, leaving only the closed flat window"
+        )
+
+
 def test_is_off_peak_without_override_dates_is_unchanged():
     """The plain DeepSeek windows with no override_dates bill a holiday peak hour at
     standard rates."""
diff --git a/tests/test_litellm/test_utils.py b/tests/test_litellm/test_utils.py
index 3fca6935d57..46ae7fd1e79 100644
--- a/tests/test_litellm/test_utils.py
+++ b/tests/test_litellm/test_utils.py
@@ -1057,6 +1057,7 @@ def test_aaamodel_prices_and_context_window_json_is_valid():
                                         "type": "array",
                                         "items": {"oneOf": [{"type": "integer"}, {"type": "string"}]},
                                     },
+                                    "override_dates": {"type": "array", "items": {"type": "string"}},
                                 },
                                 "required": ["hours_utc"],
                                 "additionalProperties": False,