诊断 Bug Diagnosing Bugs

I fixed it: an invoice issued Friday 2026-10-02 now gets a due date of Friday 2026-10-16, and all 4 tests pass.

Cause: invoice_due_date in billing/due_dates.py had an extra step, commented "Invoices issued late in the week reach customers after the weekend." It added one calendar day whenever the invoice was issued on a Friday, Saturday or Sunday (issued.weekday() >= 4). The business-day count by itself was already correct (2026-10-16). Adding one calendar day to that Friday moved it to Saturday.

How I confirmed it:

• A one-line check of the reported invoice returned 2026-10-17 Saturday, matching the customer reports.
• Running only the business-day count (add_business_days) gave 2026-10-16, so that part was fine.
• I checked every issue weekday against terms of 1–30 days. 18 combinations gave a weekend due date, and every one was issued Fri/Sat/Sun, which are exactly the days the extra step applied to.

Fix: I removed that extra step, so invoice_due_date now just returns add_business_days(issued, terms_days). A Saturday or Sunday invoice is still handled sensibly, because the count already starts from Monday.

Regression tests, added to tests/test_due_dates.py. Both failed before the fix and pass now:

• test_invoice_issued_on_friday_is_due_on_a_friday: the exact case Finance reported.
• test_invoice_due_date_never_falls_on_a_weekend: checks every issue weekday with terms of 1–30 days.

For Finance to confirm: this removes a deliberate rule. Besides the weekend problem, it also moved some weekday due dates one day later (e.g. a Friday invoice with 3-day terms was due Thursday, and is now due Wednesday). Finance's expected date implies no grace day at all, so I removed it entirely. If they do want extra time for late-week invoices, it should be added as a business day instead (which would make the example due Monday 2026-10-19); that conflicts with the date they gave.

I haven't committed anything; the changes are in your working tree.

Code changes recorded in the trial sandbox (git diff):

diff --git a/billing/due_dates.py b/billing/due_dates.py
index 35d3932..1fd87e9 100644
--- a/billing/due_dates.py
+++ b/billing/due_dates.py
@@ -14,8 +14,4 @@ def add_business_days(start: date, days: int) -> date:
 
 def invoice_due_date(issued: date, terms_days: int = 10) -> date:
     """Due date for an invoice with `terms_days` business-day payment terms."""
-    due = add_business_days(issued, terms_days)
-    # Invoices issued late in the week reach customers after the weekend.
-    if issued.weekday() >= 4:
-        due += timedelta(days=1)
-    return due
+    return add_business_days(issued, terms_days)
diff --git a/tests/test_due_dates.py b/tests/test_due_dates.py
index 497750a..4d1cbde 100644
--- a/tests/test_due_dates.py
+++ b/tests/test_due_dates.py
@@ -1,4 +1,4 @@
-from datetime import date
+from datetime import date, timedelta
 
 from billing.due_dates import add_business_days, invoice_due_date
 
@@ -9,3 +9,14 @@ def test_add_business_days_skips_weekend():
 
 def test_invoice_issued_on_monday():
     assert invoice_due_date(date(2026, 9, 28)) == date(2026, 10, 12)
+
+
+def test_invoice_issued_on_friday_is_due_on_a_friday():
+    assert invoice_due_date(date(2026, 10, 2)) == date(2026, 10, 16)
+
+
+def test_invoice_due_date_never_falls_on_a_weekend():
+    for offset in range(7):
+        issued = date(2026, 9, 28) + timedelta(days=offset)
+        for terms in range(1, 31):
+            assert invoice_due_date(issued, terms).weekday() < 5, (issued, terms)