diff --git a/src/sla_monitor/app.py b/src/sla_monitor/app.py
index d4f0657..a5ac69e 100644
--- a/src/sla_monitor/app.py
+++ b/src/sla_monitor/app.py
@@ -185,11 +185,17 @@ def _business_minutes_elapsed(since_utc, now_utc):
 
     while current < now:
         if current.weekday() < 5:
-            end_of_day = (current + timedelta(days=1)).replace(
-                hour=0, minute=0, second=0, microsecond=0
+            # Count only the business-hours window [BH_START, BH_END) on this
+            # weekday. Clamp the day's span to the portion that overlaps
+            # [current, now); overnight hours (BH_END..next BH_START) are excluded.
+            day_start = current.replace(
+                hour=BH_START, minute=0, second=0, microsecond=0
             )
-            day_end = min(end_of_day, now)
-            total += (day_end - current).total_seconds() / 60
+            day_close = current.replace(hour=BH_END, minute=0, second=0, microsecond=0)
+            span_start = max(current, day_start)
+            span_end = min(day_close, now)
+            if span_end > span_start:
+                total += (span_end - span_start).total_seconds() / 60
 
         current = (current + timedelta(days=1)).replace(
             hour=0, minute=0, second=0, microsecond=0