diff --git a/src/copilot_usage/models.py b/src/copilot_usage/models.py
index 2e14ee8..a8d84ed 100644
--- a/src/copilot_usage/models.py
+++ b/src/copilot_usage/models.py
@@ -124,7 +124,7 @@ class AssistantMessageData(BaseModel):
     reasoningText: str | None = None
     reasoningOpaque: str | None = None
     toolRequests: list[dict[str, object]] = Field(
-        default_factory=lambda: list[dict[str, object]]()
+        default_factory=list[dict[str, object]]
     )
 
 
diff --git a/src/copilot_usage/report.py b/src/copilot_usage/report.py
index 0d55787..2ca8c64 100644
--- a/src/copilot_usage/report.py
+++ b/src/copilot_usage/report.py
@@ -111,6 +111,18 @@ def _estimated_output_tokens(session: SessionSummary) -> int:
     return sum(m.usage.outputTokens for m in session.model_metrics.values())
 
 
+def _format_session_running_time(session: SessionSummary) -> str:
+    """Return a human-readable running time for *session*.
+
+    Returns ``"—"`` when the session has no ``start_time``.  Otherwise
+    delegates to :func:`_format_elapsed_since`, preferring
+    ``last_resume_time`` over ``start_time``.
+    """
+    if not session.start_time:
+        return "—"
+    return _format_elapsed_since(session.last_resume_time or session.start_time)
+
+
 def render_live_sessions(sessions: list[SessionSummary]) -> None:
     """Render overview of active sessions only.
 
@@ -144,11 +156,7 @@ def render_live_sessions(sessions: list[SessionSummary]) -> None:
         short_id = s.session_id[:8] if s.session_id else "—"
         name = s.name or "—"
         model = s.model or "—"
-        running = (
-            _format_elapsed_since(s.last_resume_time or s.start_time)
-            if s.start_time
-            else "—"
-        )
+        running = _format_session_running_time(s)
         messages = str(s.user_messages)
         tokens = f"{_estimated_output_tokens(s):,}"
         cwd = s.cwd or "—"
@@ -580,10 +588,9 @@ def _render_totals(console: Console, sessions: list[SessionSummary]) -> None:
     total_duration = sum(s.total_api_duration_ms for s in sessions)
     total_sessions = len(sessions)
 
-    total_output = 0
-    for s in sessions:
-        for mm in s.model_metrics.values():
-            total_output += mm.usage.outputTokens
+    total_output = sum(
+        mm.usage.outputTokens for s in sessions for mm in s.model_metrics.values()
+    )
 
     pr_label = "premium request" if total_premium == 1 else "premium requests"
     session_label = "session" if total_sessions == 1 else "sessions"
@@ -788,11 +795,7 @@ def _render_active_section(
     for s in active:
         name = s.name or s.session_id[:12]
         model = s.model or "—"
-        running = (
-            _format_elapsed_since(s.last_resume_time or s.start_time)
-            if s.start_time
-            else "—"
-        )
+        running = _format_session_running_time(s)
 
         table.add_row(
             name,
@@ -847,9 +850,9 @@ def render_cost_view(
     for premium and the active model calls / output tokens.
     """
     console = target_console or Console()
-    sessions = _filter_sessions(sessions, since, until)
+    filtered = _filter_sessions(sessions, since, until)
 
-    if not sessions:
+    if not filtered:
         console.print("[yellow]No sessions found.[/yellow]")
         return
 
@@ -866,7 +869,7 @@ def render_cost_view(
     grand_model_calls = 0
     grand_output = 0
 
-    for s in sessions:
+    for s in filtered:
         name = s.name or s.session_id[:12]
         model_calls_display = str(s.model_calls)
 
diff --git a/tests/copilot_usage/test_report.py b/tests/copilot_usage/test_report.py
index 7bfa26e..447932d 100644
--- a/tests/copilot_usage/test_report.py
+++ b/tests/copilot_usage/test_report.py
@@ -27,6 +27,7 @@
     _filter_sessions,
     _format_detail_duration,
     _format_relative_time,
+    _format_session_running_time,
     format_duration,
     format_tokens,
     render_cost_view,
@@ -1699,3 +1700,32 @@ def test_one_session_one_premium_request(self) -> None:
         # "session" appears without trailing 's'
         stripped = output.replace("sessions", "")
         assert "session" in stripped
+
+
+# ---------------------------------------------------------------------------
+# Issue #66 — _format_session_running_time helper
+# ---------------------------------------------------------------------------
+
+
+class TestFormatSessionRunningTime:
+    """Tests for the extracted _format_session_running_time helper."""
+
+    def test_returns_dash_when_no_start_time(self) -> None:
+        session = SessionSummary(session_id="no-start")
+        assert _format_session_running_time(session) == "—"
+
+    def test_uses_start_time_when_no_resume(self) -> None:
+        start = datetime.now(tz=UTC) - timedelta(minutes=5, seconds=30)
+        session = SessionSummary(session_id="s1", start_time=start)
+        result = _format_session_running_time(session)
+        assert "5m" in result
+
+    def test_uses_last_resume_time_over_start_time(self) -> None:
+        start = datetime.now(tz=UTC) - timedelta(hours=2)
+        resume = datetime.now(tz=UTC) - timedelta(minutes=3)
+        session = SessionSummary(
+            session_id="s2", start_time=start, last_resume_time=resume
+        )
+        result = _format_session_running_time(session)
+        assert "3m" in result
+        assert "2h" not in result