From f694ec3aceab8e2385deaf143ffe3c7e42cd30a2 Mon Sep 17 00:00:00 2001
From: ReturnKartikey <kartikeynegi2000.work@gmail.com>
Date: Thu, 8 Oct 2026 01:08:20 +0530
Subject: [PATCH] Fix subsecond formatting for pre-epoch local_time (#4975)

---
 include/fmt/chrono.h | 9 +++++++++
 test/chrono-test.cc  | 8 ++++++++
 2 files changed, 17 insertions(+)

diff --git a/include/fmt/chrono.h b/include/fmt/chrono.h
index 19571164..a1c13483 100644
--- a/include/fmt/chrono.h
+++ b/include/fmt/chrono.h
@@ -2245,6 +2245,15 @@ struct formatter<local_time<Duration>, Char>
     }
     auto subsecs =
         detail::duration_cast<Duration>(time_since_epoch - seconds_since_epoch);
+    if (subsecs.count() < 0) {
+      auto second = detail::duration_cast<Duration>(std::chrono::seconds(1));
+      if (t.tm_sec != 0) {
+        --t.tm_sec;
+      } else {
+        t = gmtime(seconds_since_epoch.count() - 1);
+      }
+      subsecs += second;
+    }
     return formatter<std::tm, Char>::do_format(t, ctx, &subsecs);
   }
 };
diff --git a/test/chrono-test.cc b/test/chrono-test.cc
index 818ec42f..d9ea18a1 100644
--- a/test/chrono-test.cc
+++ b/test/chrono-test.cc
@@ -332,6 +332,14 @@ TEST(chrono_test, local_time) {
                    fmt::format_error, "no timezone");
   EXPECT_THROW_MSG((void)fmt::format(fmt::runtime("{:%Z}"), time),
                    fmt::format_error, "no timezone");
+
+  auto epoch = fmt::local_time<std::chrono::milliseconds>();
+  auto d = std::chrono::milliseconds(250);
+  EXPECT_EQ(fmt::format("{:%S}", epoch - d), "59.750");
+
+  auto pre_epoch = fmt::local_time<std::chrono::milliseconds>(
+      std::chrono::milliseconds(-14398877));
+  EXPECT_EQ(fmt::format("{:%S}", pre_epoch), "01.123");
 }
 
 template <typename T, FMT_ENABLE_IF(fmt::detail::has_tm_gmtoff<T>::value)>