diff --git a/src/copilot_usage/parser.py b/src/copilot_usage/parser.py
index bd2f551..b5399b9 100644
--- a/src/copilot_usage/parser.py
+++ b/src/copilot_usage/parser.py
@@ -8,6 +8,7 @@
 from __future__ import annotations
 
 import json
+from datetime import UTC, datetime
 from pathlib import Path
 
 from loguru import logger
@@ -29,6 +30,16 @@
 _DEFAULT_BASE: Path = Path.home() / ".copilot" / "session-state"
 _CONFIG_PATH: Path = Path.home() / ".copilot" / "config.json"
 
+_RESUME_INDICATOR_TYPES: frozenset[str] = frozenset(
+    {
+        EventType.SESSION_RESUME,
+        EventType.USER_MESSAGE,
+        EventType.ASSISTANT_MESSAGE,
+    }
+)
+
+_EPOCH: datetime = datetime.min.replace(tzinfo=UTC)
+
 
 def _infer_model_from_metrics(metrics: dict[str, ModelMetrics]) -> str | None:
     """Pick a model name from *metrics* when ``currentModel`` is absent.
@@ -234,12 +245,6 @@ def build_session_summary(
     name = _extract_session_name(session_dir) if session_dir else None
 
     # --- Detect resumed session (events after last shutdown) --------------
-    _RESUME_INDICATOR_TYPES: set[str] = {
-        EventType.SESSION_RESUME,
-        EventType.USER_MESSAGE,
-        EventType.ASSISTANT_MESSAGE,
-    }
-
     session_resumed = False
     post_shutdown_output_tokens = 0
     post_shutdown_turn_starts = 0
@@ -365,10 +370,8 @@ def get_all_sessions(base_path: Path | None = None) -> list[SessionSummary]:
         summary.events_path = events_path
         summaries.append(summary)
 
-    def _sort_key(s: SessionSummary) -> str:
-        if s.start_time is None:
-            return ""
-        return s.start_time.isoformat()
+    def _sort_key(s: SessionSummary) -> datetime:
+        return s.start_time if s.start_time is not None else _EPOCH
 
     summaries.sort(key=_sort_key, reverse=True)
     return summaries
diff --git a/src/copilot_usage/pricing.py b/src/copilot_usage/pricing.py
index 03ef94a..24d9f0b 100644
--- a/src/copilot_usage/pricing.py
+++ b/src/copilot_usage/pricing.py
@@ -83,6 +83,7 @@ def _tier_from_multiplier(m: float) -> PricingTier:
     "gpt-5.3-codex": 1.0,
     "gpt-5.1-codex-max": 1.0,
     "gpt-5.1-codex-mini": 0.33,
+    "gpt-5.4-mini": 0.0,
     "gpt-5-mini": 0.0,
     "gpt-4.1": 0.0,
     # Gemini -----------------------------------------------------------------
diff --git a/tests/copilot_usage/test_pricing.py b/tests/copilot_usage/test_pricing.py
index 0c59c28..f79d864 100644
--- a/tests/copilot_usage/test_pricing.py
+++ b/tests/copilot_usage/test_pricing.py
@@ -51,6 +51,7 @@ def test_registry_not_empty(self) -> None:
             ("gpt-5.1-codex-max", 1.0),
             ("gpt-4.1", 0.0),
             ("gpt-5-mini", 0.0),
+            ("gpt-5.4-mini", 0.0),
             ("gemini-3-pro-preview", 1.0),
         ],
     )
@@ -64,6 +65,7 @@ def test_known_multipliers(self, model: str, expected_mult: float) -> None:
             ("claude-sonnet-4.6", PricingTier.STANDARD),
             ("claude-haiku-4.5", PricingTier.LIGHT),
             ("gpt-5-mini", PricingTier.LIGHT),
+            ("gpt-5.4-mini", PricingTier.LIGHT),
         ],
     )
     def test_known_tiers(self, model: str, expected_tier: PricingTier) -> None: