From 12ffc09458f3b6c4bc1b69a1688d7db8806761d2 Mon Sep 17 00:00:00 2001
From: "carpentry-heartbeat[bot]"
 <carpentry-heartbeat[bot]@users.noreply.github.com>
Date: Sat, 15 Aug 2026 00:26:48 +0200
Subject: [PATCH] Don't apply the am/pm shift to an unparsed hour in strptime

The hour accumulator starts at the sentinel -1, and the am/pm adjustment
ran before that sentinel was checked. For a format with %p but no %H or
%I, PM took the (< hr 12) branch and turned -1 into 11, so the sentinel
check on the next line no longer matched and the caller was handed a
fabricated 11:00 instead of Nothing:

    (Datetime.strptime "PM 2024-03-15" "%p %Y-%m-%d") -> hours = (Just 11)

AM was unaffected, since (= hr 12) is false for -1.

Skip the adjustment when the hour was never parsed, so hours comes back
as Nothing like every other unparsed field in that same let.
---
 test/time.carp | 25 +++++++++++++++++++++++++
 time.carp      |  1 +
 2 files changed, 26 insertions(+)

diff --git a/test/time.carp b/test/time.carp
index a3421ed..aa0b114 100644
--- a/test/time.carp
+++ b/test/time.carp
@@ -1152,6 +1152,31 @@
       (Result.Error _) -2)
     "strptime %I %p parses 12 PM as 12")
 
+  (assert-equal test
+    -1
+    (match (Datetime.strptime "PM 2024-03-15" "%p %Y-%m-%d")
+      (Result.Success dt) (Maybe.from @(Datetime.hours &dt) -1)
+      (Result.Error _) -2)
+    "strptime %p with no hour specifier leaves the hour unset (PM)")
+  (assert-equal test
+    -1
+    (match (Datetime.strptime "AM 2024-03-15" "%p %Y-%m-%d")
+      (Result.Success dt) (Maybe.from @(Datetime.hours &dt) -1)
+      (Result.Error _) -2)
+    "strptime %p with no hour specifier leaves the hour unset (AM)")
+  (assert-equal test
+    15
+    (match (Datetime.strptime "03 PM 2024-03-15" "%I %p %Y-%m-%d")
+      (Result.Success dt) (Maybe.from @(Datetime.hours &dt) -1)
+      (Result.Error _) -2)
+    "strptime %I %p parses 03 PM as 15")
+  (assert-equal test
+    7
+    (match (Datetime.strptime "07 2024-03-15" "%H %Y-%m-%d")
+      (Result.Success dt) (Maybe.from @(Datetime.hours &dt) -1)
+      (Result.Error _) -2)
+    "strptime %H parses the hour with no am/pm marker")
+
   (assert-equal test
     123456789
     (match (Datetime.strptime "14:30:00.123456789" "%H:%M:%S.%n")
diff --git a/time.carp b/time.carp
index 448b8f5..cf44762 100644
--- a/time.carp
+++ b/time.carp
@@ -1090,6 +1090,7 @@ Example:
           (Result.Error e) (Result.Error e)
           (Result.Success d)
             (let [final-hr (cond
+                    (= hr -1) hr
                     (= ampm 2) (if (< hr 12) (+ hr 12) hr)
                     (= ampm 1) (if (= hr 12) 0 hr)
                     hr)