[fix][kernel] fix timegm month overflow calculation

When days exceed the number of days in the current month, the original
code incorrectly used __spm[t->tm_mon] as the monthly offset, causing
wrong date results after crossing month boundaries (e.g. adding 420 days
to 2026/08/05 gave 2027/03/01 instead of 2027/08/29).

Fix the overflow logic to use (__spm[mon+1] - __spm[mon]) with proper
leap-year adjustment for February.

Close #11686

Signed-off-by: Hui Su <3164683437@qq.com>

Author: liulangrenaaa

components/libc/compilers/common/ctime.c

@@ -396,6 +396,7 @@ time_t timegm(struct tm * const t)
     time_t i;
     time_t years;
 
+    /* 归一化时间结构中的进位字段。 */
     if(t == RT_NULL)
     {
         rt_set_errno(EFAULT);
@@ -422,13 +423,9 @@ time_t timegm(struct tm * const t)
         t->tm_year += t->tm_mon / 12;
         t->tm_mon %= 12;
     }
-    while (t->tm_mday > __spm[1 + t->tm_mon])
+    while (t->tm_mday > __spm[t->tm_mon + 1] - __spm[t->tm_mon] + (__isleap(t->tm_year + 1900) && t->tm_mon == 1))
     {
-        if (t->tm_mon == 1 && __isleap(t->tm_year + 1900))
-        {
-            --t->tm_mday;
-        }
-        t->tm_mday -= __spm[t->tm_mon];
+        t->tm_mday -= __spm[t->tm_mon + 1] - __spm[t->tm_mon] + (__isleap(t->tm_year + 1900) && t->tm_mon == 1);
         ++t->tm_mon;
         if (t->tm_mon > 11)
         {

src/utest/Kconfig

@@ -10,6 +10,11 @@ menu "Kernel Core"
         default n
         depends on RT_USING_SMALL_MEM
 
+    config RT_UTEST_TIME
+        bool "Time Conversion Test"
+        default n
+        depends on RT_USING_POSIX_CLOCK
+
     config RT_UTEST_OBJECT
         select RT_USING_DEVICE
         select RT_USING_SEMAPHORE

src/utest/SConscript

@@ -14,6 +14,9 @@ if GetDepend(['RT_UTEST_MEMHEAP']):
 if GetDepend(['RT_UTEST_SMALL_MEM']):
     src += ['mem_tc.c']
 
+if GetDepend(['RT_UTEST_TIME']):
+    src += ['time_tc.c']
+
 if GetDepend(['RT_UTEST_SLAB']):
     src += ['slab_tc.c']
 
@@ -73,4 +76,3 @@ for item in list:
         group = group + SConscript(os.path.join(item, 'SConscript'))
 
 Return('group')
-

src/utest/time_tc.c

@@ -0,0 +1,90 @@
+/*
+ * Copyright (c) 2006-2026, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Test Case Name: POSIX Time Conversion Test
+ *
+ * Test Objectives:
+ * - Verify that timegm normalizes dates across month and year boundaries.
+ * - Verify leap-year February handling.
+ * - Prevent regression of GitHub Issue #11686.
+ */
+
+#include <rtthread.h>
+#include <sys/time.h>
+#include "utest.h"
+
+/* 验证跨越普通月份边界时的日期归一化。 */
+static void timegm_month_boundary_test(void)
+{
+    struct tm time_value = { 0 };
+
+    time_value.tm_year = 123;
+    time_value.tm_mon = 0;
+    time_value.tm_mday = 32;
+
+    timegm(&time_value);
+
+    uassert_int_equal(time_value.tm_year, 123);
+    uassert_int_equal(time_value.tm_mon, 1);
+    uassert_int_equal(time_value.tm_mday, 1);
+}
+
+/* 验证闰年二月的日期归一化。 */
+static void timegm_leap_year_test(void)
+{
+    struct tm time_value = { 0 };
+
+    time_value.tm_year = 124;
+    time_value.tm_mon = 0;
+    time_value.tm_mday = 60;
+
+    timegm(&time_value);
+
+    uassert_int_equal(time_value.tm_year, 124);
+    uassert_int_equal(time_value.tm_mon, 1);
+    uassert_int_equal(time_value.tm_mday, 29);
+}
+
+/* 验证大日期偏移场景的日期归一化。 */
+static void timegm_large_day_test(void)
+{
+    struct tm time_value = { 0 };
+
+    time_value.tm_year = 126;
+    time_value.tm_mon = 7;
+    time_value.tm_mday = 425;
+    time_value.tm_hour = 8;
+
+    timegm(&time_value);
+
+    uassert_int_equal(time_value.tm_year, 127);
+    uassert_int_equal(time_value.tm_mon, 8);
+    uassert_int_equal(time_value.tm_mday, 29);
+    uassert_int_equal(time_value.tm_hour, 8);
+}
+
+/* 初始化时间转换测试。 */
+static rt_err_t utest_tc_init(void)
+{
+    return RT_EOK;
+}
+
+/* 清理时间转换测试。 */
+static rt_err_t utest_tc_cleanup(void)
+{
+    return RT_EOK;
+}
+
+/* 执行时间转换测试用例。 */
+static void testcase(void)
+{
+    UTEST_UNIT_RUN(timegm_month_boundary_test);
+    UTEST_UNIT_RUN(timegm_leap_year_test);
+    UTEST_UNIT_RUN(timegm_large_day_test);
+}
+
+UTEST_TC_EXPORT(testcase, "core.time", utest_tc_init, utest_tc_cleanup, 10);