diff --git a/src/wf_scheduling/history.py b/src/wf_scheduling/history.py
index 9b374662..1209bdf8 100644
--- a/src/wf_scheduling/history.py
+++ b/src/wf_scheduling/history.py
@@ -16,11 +16,15 @@ from datetime import datetime, timezone
 from typing import Any, Protocol
 
 from wf_scheduling.models import OccurrenceKind, OccurrenceRecord
-from wf_scheduling.occurrences import occurrence_id
+from wf_scheduling.occurrences import _require_aware, occurrence_id
 
 UTC = timezone.utc
 
-TERMINAL_KINDS: tuple[str, str, str] = ("completed", "interrupted", "failed")
+TERMINAL_KINDS: tuple[OccurrenceKind, ...] = (
+    "completed",
+    "interrupted",
+    "failed",
+)
 
 
 @dataclass(frozen=True)
@@ -53,7 +57,7 @@ class HistoryRecorder(Protocol):
         self,
         schedule_id: str,
         run_id: str,
-        kind: str,
+        kind: OccurrenceKind,
         checkpoint_id: str | None = None,
     ) -> bool:
         """Whether this exact stopped result was already reconciled."""
@@ -62,15 +66,32 @@ class HistoryRecorder(Protocol):
 
 def entry_occurrence_id(entry: HistoryEntry, created: datetime) -> str:
     """Derive the history occurrence id with the scheduler's standing rules."""
+    interval = _normalized_interval(entry)
     if entry.resolved_at is not None:
         return occurrence_id(entry.schedule_id, entry.resolved_at)
-    if entry.interval_start is not None and entry.interval_end is not None:
-        start = entry.interval_start.isoformat()
-        end = entry.interval_end.isoformat()
-        return f"{entry.schedule_id}|summary|{start}|{end}"
+    if interval is not None:
+        start, end = interval
+        return f"{entry.schedule_id}|summary|{start.isoformat()}|{end.isoformat()}"
     return f"{entry.schedule_id}|summary|{created.isoformat()}"
 
 
+def _normalized_interval(
+    entry: HistoryEntry,
+) -> tuple[datetime, datetime] | None:
+    """Validate and normalize an optional summary interval to UTC."""
+    start, end = entry.interval_start, entry.interval_end
+    if (start is None) != (end is None):
+        raise ValueError("interval_start and interval_end must be provided together")
+    if start is None:
+        assert end is None
+        return None
+    assert end is not None
+    return (
+        _require_aware(start, op="interval_start").astimezone(UTC),
+        _require_aware(end, op="interval_end").astimezone(UTC),
+    )
+
+
 class FileScheduleHistoryRecorder:
     """History recorder backed by a file schedule store."""
 
@@ -81,6 +102,7 @@ class FileScheduleHistoryRecorder:
         created = (
             entry.created_at if entry.created_at is not None else datetime.now(UTC)
         )
+        interval = _normalized_interval(entry)
         self.schedule_store.append_history(
             OccurrenceRecord(
                 schedule_id=entry.schedule_id,
@@ -93,8 +115,8 @@ class FileScheduleHistoryRecorder:
                 admitted_at=entry.admitted_at,
                 started_at=entry.started_at,
                 checkpoint_id=entry.checkpoint_id,
-                interval_start=entry.interval_start,
-                interval_end=entry.interval_end,
+                interval_start=interval[0] if interval is not None else None,
+                interval_end=interval[1] if interval is not None else None,
                 interval_count=entry.interval_count,
                 created_at=created,
             )
@@ -104,7 +126,7 @@ class FileScheduleHistoryRecorder:
         self,
         schedule_id: str,
         run_id: str,
-        kind: str,
+        kind: OccurrenceKind,
         checkpoint_id: str | None = None,
     ) -> bool:
         return self.schedule_store.has_history_entry(
diff --git a/src/wf_scheduling/store.py b/src/wf_scheduling/store.py
index bdb5d900..4b62974b 100644
--- a/src/wf_scheduling/store.py
+++ b/src/wf_scheduling/store.py
@@ -26,7 +26,13 @@ from pathlib import Path
 from threading import RLock
 from typing import Any, cast
 
-from .models import OccurrenceRecord, PendingCandidate, Schedule, ensure_schedule_id
+from .models import (
+    OccurrenceKind,
+    OccurrenceRecord,
+    PendingCandidate,
+    Schedule,
+    ensure_schedule_id,
+)
 
 UTC = timezone.utc
 
@@ -48,16 +54,18 @@ def schedule_store_transaction(store: object) -> Iterator[None]:
 
 
 def _history_identity(record: OccurrenceRecord) -> tuple[object, ...]:
-    """Return fields that identify one durable occurrence transition."""
+    """Return the canonical fields that identify one history transition.
+
+    ``occurrence_id`` already contains the UTC-normalized resolved instant or
+    summary interval. Do not compare the original datetime fields as well:
+    equivalent offsets must remain one idempotent history entry.
+    """
     return (
         record.schedule_id,
         record.occurrence_id,
         record.kind,
-        record.resolved_at,
         record.run_id,
         record.checkpoint_id,
-        record.interval_start,
-        record.interval_end,
     )
 
 
@@ -237,7 +245,7 @@ class FileScheduleStore:
         schedule_id: str,
         *,
         run_id: str,
-        kind: str,
+        kind: OccurrenceKind,
         checkpoint_id: str | None = None,
     ) -> bool:
         """Whether this exact stopped result already has a history entry.