diff --git a/backend/business_hours/calendar.py b/backend/business_hours/calendar.py
index 03ee98ae..72b0f65b 100644
--- a/backend/business_hours/calendar.py
+++ b/backend/business_hours/calendar.py
@@ -91,15 +91,20 @@ def add_business_hours(start_dt: datetime, hours: float, calendar) -> datetime:
         day_open = datetime.combine(date_local, open_t, tzinfo=tz)
         day_close = datetime.combine(date_local, close_t, tzinfo=tz)
 
-        if cur >= day_close:
+        # Compare in UTC: same-tzinfo aware datetimes compare by wall clock and
+        # ignore ``fold``, which breaks inside the repeated fall-back hour.
+        day_close_utc = day_close.astimezone(_UTC)
+        cur_utc = cur.astimezone(_UTC)
+        if cur_utc >= day_close_utc:
             cur = _next_midnight(tz, cur)
             continue
-        if cur < day_open:
+        if cur_utc < day_open.astimezone(_UTC):
             cur = day_open
+            cur_utc = cur.astimezone(_UTC)
 
-        available = day_close - cur
+        available = day_close_utc - cur_utc
         if remaining <= available:
-            return (cur + remaining).astimezone(out_tz)
+            return (cur_utc + remaining).astimezone(out_tz)
         remaining -= available
         cur = _next_midnight(tz, cur)
 
diff --git a/backend/business_hours/tests/test_calendar.py b/backend/business_hours/tests/test_calendar.py
index 030c0aed..565ffd29 100644
--- a/backend/business_hours/tests/test_calendar.py
+++ b/backend/business_hours/tests/test_calendar.py
@@ -81,6 +81,62 @@ class TestAddBusinessHours:
         # 1h Fri 4-5pm + 8h Monday 9-5 = Mon 3/9 5pm NY (post-DST EDT).
         assert result.astimezone(_NY) == datetime(2026, 3, 9, 17, 0, tzinfo=_NY)
 
+    @pytest.mark.parametrize(
+        "start, real_hours",
+        [
+            # Spring forward: the 00:00-08:00 window is 7 real hours, so the
+            # 8th hour lands at 01:00 the next day (24 real hours later).
+            (datetime(2025, 3, 9, 0, 0, tzinfo=_NY), 24),
+            # Fall back: the window is 9 real hours, so 8h end at 07:00 EST.
+            (datetime(2025, 11, 2, 0, 0, tzinfo=_NY), 8),
+        ],
+    )
+    def test_dst_transition_inside_window(self, org_a, start, real_hours):
+        from datetime import time
+
+        from business_hours.models import BusinessCalendar
+
+        days = "monday tuesday wednesday thursday friday saturday sunday".split()
+        cal = BusinessCalendar.objects.create(
+            org=org_a,
+            name="Overnight",
+            timezone="America/New_York",
+            is_default=False,
+            **{
+                f"{d}_{k}": t
+                for d in days
+                for k, t in (("open", time(0, 0)), ("close", time(8, 0)))
+            },
+        )
+        result = add_business_hours(start, 8, cal)
+        elapsed = (
+            result.astimezone(_UTC) - start.astimezone(_UTC)
+        ).total_seconds() / 3600
+        assert elapsed == real_hours
+
+    def test_start_in_repeated_hour_after_close(self, org_a):
+        from datetime import time
+
+        from business_hours.models import BusinessCalendar
+
+        days = "monday tuesday wednesday thursday friday saturday sunday".split()
+        cal = BusinessCalendar.objects.create(
+            org=org_a,
+            name="Early close",
+            timezone="America/New_York",
+            is_default=False,
+            **{
+                f"{d}_{k}": t
+                for d in days
+                for k, t in (("open", time(0, 0)), ("close", time(1, 45)))
+            },
+        )
+        # 06:30Z on 2025-11-02 is 01:30 EST (second pass); the 01:45 EDT close
+        # (05:45Z) already passed, so work resumes at the next midnight.
+        start = datetime(2025, 11, 2, 6, 30, tzinfo=_UTC)
+        result = add_business_hours(start, 1, cal)
+        assert result.astimezone(_NY) == datetime(2025, 11, 3, 1, 0, tzinfo=_NY)
+
     def test_naive_input_treated_as_utc(self, calendar_a):
         start_naive = datetime(2026, 5, 11, 10, 0)
         result = add_business_hours(start_naive, 2, calendar_a)