--- a/app.py
+++ b/app.py
@@ -1,4 +1,12 @@
 from datetime import timedelta
 
 def due_date(start, days):
-    return start + timedelta(days=days)
+    if days < 0:
+        raise ValueError('negative days')
+    cursor = start
+    remaining = days
+    while remaining:
+        cursor += timedelta(days=1)
+        if cursor.weekday() < 5:
+            remaining -= 1
+    return cursor