[fix][libc] fix timegm month overflow calculation

The original overflow logic used __spm[tm_mon] as the current month's
length when normalizing tm_mday. This produced incorrect dates after
crossing month boundaries and left zero or negative days unnormalized.

Use the difference between adjacent cumulative day offsets with
leap-year handling. Normalize overflow in tm_sec, tm_min, tm_hour,
tm_mday, and tm_mon to keep the affected broken-down fields normalized.

Add regression tests for month boundaries, leap years, large offsets,
zero and negative days, negative time fields, combined overflow, and
boundary carry cases.

Close RT-Thread#11686

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

Author: liulangrenaaa

.github/utest/configs/components/libc.cfg

@@ -0,0 +1,5 @@
+# dependencies
+CONFIG_RT_CONSOLEBUF_SIZE=1024
+CONFIG_RT_USING_CI_ACTION=y
+CONFIG_RT_USING_POSIX_CLOCK=y
+CONFIG_RT_UTEST_LIBC_TIME=y

.github/workflows/utest_auto_run.yml

@@ -150,6 +150,8 @@ jobs:
             config_file: "components/sal.cfg"
           - platform: { UTEST: "A9",            RTT_BSP: "bsp/qemu-vexpress-a9",      QEMU_ARCH: "arm",     QEMU_MACHINE: "vexpress-a9", SD_FILE: "sd.bin", KERNEL: "standard", "SMP_RUN":"" }
             config_file: "components/dfs.cfg"
+          - platform: { UTEST: "A9",            RTT_BSP: "bsp/qemu-vexpress-a9",      QEMU_ARCH: "arm",     QEMU_MACHINE: "vexpress-a9", SD_FILE: "sd.bin", KERNEL: "standard", "SMP_RUN":"" }
+            config_file: "components/libc.cfg"
 
     env:
       TEST_QEMU_ARCH: ${{ matrix.platform.QEMU_ARCH }}

components/libc/compilers/common/ctime.c

@@ -390,45 +390,83 @@ int stime(const time_t *t)
 }
 RTM_EXPORT(stime);
 
+/* Normalize the time fields and convert the result to a UTC timestamp. */
 time_t timegm(struct tm * const t)
 {
     time_t day;
     time_t i;
     time_t years;
+    int days_in_month;
 
     if(t == RT_NULL)
     {
         rt_set_errno(EFAULT);
         return (time_t)-1;
     }
 
-    if (t->tm_sec > 60)         /* seconds after the minute - [0, 60] including leap second */
+    if (t->tm_sec < 0 || t->tm_sec > 60) /* seconds after the minute - [0, 60] including leap second */
     {
         t->tm_min += t->tm_sec / 60;
         t->tm_sec %= 60;
+        if (t->tm_sec < 0)
+        {
+            t->tm_sec += 60;
+            --t->tm_min;
+        }
     }
-    if (t->tm_min >= 60)        /* minutes after the hour - [0, 59] */
+    if (t->tm_min < 0 || t->tm_min >= 60) /* minutes after the hour - [0, 59] */
     {
         t->tm_hour += t->tm_min / 60;
         t->tm_min %= 60;
+        if (t->tm_min < 0)
+        {
+            t->tm_min += 60;
+            --t->tm_hour;
+        }
     }
-    if (t->tm_hour >= 24)       /* hours since midnight - [0, 23] */
+    if (t->tm_hour < 0 || t->tm_hour >= 24) /* hours since midnight - [0, 23] */
     {
         t->tm_mday += t->tm_hour / 24;
         t->tm_hour %= 24;
+        if (t->tm_hour < 0)
+        {
+            t->tm_hour += 24;
+            --t->tm_mday;
+        }
     }
-    if (t->tm_mon >= 12)        /* months since January - [0, 11] */
+    if (t->tm_mon < 0 || t->tm_mon >= 12) /* months since January - [0, 11] */
     {
         t->tm_year += t->tm_mon / 12;
         t->tm_mon %= 12;
+        if (t->tm_mon < 0)
+        {
+            t->tm_mon += 12;
+            --t->tm_year;
+        }
     }
-    while (t->tm_mday > __spm[1 + t->tm_mon])
+    while (t->tm_mday <= 0)
     {
-        if (t->tm_mon == 1 && __isleap(t->tm_year + 1900))
+        if (t->tm_mon == 0)
         {
-            --t->tm_mday;
+            t->tm_mon = 11;
+            --t->tm_year;
+        }
+        else
+        {
+            --t->tm_mon;
+        }
+        t->tm_mday += __spm[t->tm_mon + 1] - __spm[t->tm_mon] +
+                      (__isleap(t->tm_year + 1900) && t->tm_mon == 1);
+    }
+    while (1)
+    {
+        days_in_month = __spm[t->tm_mon + 1] - __spm[t->tm_mon] +
+                        (__isleap(t->tm_year + 1900) && t->tm_mon == 1);
+        if (t->tm_mday <= days_in_month)
+        {
+            break;
         }
-        t->tm_mday -= __spm[t->tm_mon];
+        t->tm_mday -= days_in_month;
         ++t->tm_mon;
         if (t->tm_mon > 11)
         {

components/libc/compilers/common/utest/Kconfig

@@ -0,0 +1,8 @@
+menu "C Library Test"
+
+config RT_UTEST_LIBC_TIME
+    bool "Time Conversion Test"
+    default n
+    depends on RT_USING_POSIX_CLOCK
+
+endmenu

components/libc/compilers/common/utest/SConscript

@@ -0,0 +1,13 @@
+Import('rtconfig')
+from building import *
+
+cwd     = GetCurrentDir()
+src     = []
+CPPPATH = [cwd]
+
+if GetDepend(['RT_UTEST_LIBC_TIME']):
+    src += ['time_tc.c']
+
+group = DefineGroup('utestcases', src, depend = ['RT_USING_UTESTCASES'], CPPPATH = CPPPATH)
+
+Return('group')