From fad346a071323d3a9f5b595b966dabed4fc8c8ba Mon Sep 17 00:00:00 2001
From: Pierre Kancir <pierre.kancir.emn@gmail.com>
Date: Tue, 25 Aug 2026 18:01:16 +0200
Subject: [PATCH] AP_Common: update ap_mktime

---
 libraries/AP_Common/tests/test_time.cpp | 25 ++++++++++
 libraries/AP_Common/time.cpp            | 62 +++++++++++++------------
 2 files changed, 58 insertions(+), 29 deletions(-)

diff --git a/libraries/AP_Common/tests/test_time.cpp b/libraries/AP_Common/tests/test_time.cpp
index 2abd3d4a6b1..56ab63dc92e 100644
--- a/libraries/AP_Common/tests/test_time.cpp
+++ b/libraries/AP_Common/tests/test_time.cpp
@@ -111,6 +111,31 @@ TEST(ap_mktime, MonthLoopRollover)
     struct tm t = make_tm(1970, 1, 1, 0, 0, 0);
     t.tm_mon = 12; // 13 iterations: Dec wraps m→0, y→1971, then Jan added
     EXPECT_EQ((time_t)(365 * 86400), ap_mktime(&t));
+
+    // tm_mon=14 is the largest a caller can produce: AP_Filesystem_FATFS
+    // reads a 4-bit month field and subtracts one, so 0x0f arrives here
+    // as 14.  Fourteen months on from 1 January 1970 is 1 March 1971:
+    // all of 1970, then January and February.
+    t.tm_mon = 14;
+    EXPECT_EQ((time_t)((365 + 31 + 28) * 86400), ap_mktime(&t));
+}
+
+// Every caller computes tm_mon as "month - 1" from an external date
+// field, so a source reporting month 0 arrives here as -1.  Such a
+// month must contribute nothing rather than index off the front of the
+// month table.
+TEST(ap_mktime, NegativeMonth)
+{
+    struct tm jan1 = make_tm(1970, 1, 1, 0, 0, 0);
+    struct tm neg = jan1;
+    neg.tm_mon = -1;
+    EXPECT_EQ(ap_mktime(&jan1), ap_mktime(&neg));
+
+    // and it must not disturb the rest of the fields
+    struct tm neg_later = make_tm(2024, 1, 15, 6, 7, 8);
+    neg_later.tm_mon = -1;
+    struct tm jan15 = make_tm(2024, 1, 15, 6, 7, 8);
+    EXPECT_EQ(ap_mktime(&jan15), ap_mktime(&neg_later));
 }
 
 AP_GTEST_MAIN()
diff --git a/libraries/AP_Common/time.cpp b/libraries/AP_Common/time.cpp
index 81f81b0cfdc..35589de247b 100644
--- a/libraries/AP_Common/time.cpp
+++ b/libraries/AP_Common/time.cpp
@@ -1,44 +1,48 @@
 #include "time.h"
 
+#include <stdint.h>
+
 /*
-  mktime replacement from Samba
+  mktime replacement, originally from Samba
  */
 time_t ap_mktime(const struct tm *t)
 {
-    time_t epoch = 0;
-    int n;
-    int mon [] = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 }, y, m, i;
-    const unsigned MINUTE = 60;
-    const unsigned HOUR = 60*MINUTE;
-    const unsigned DAY = 24*HOUR;
-    const unsigned YEAR = 365*DAY;
+    // days elapsed from 1 January to the 1st of each month, common year
+    static const uint16_t cumdays[12] = {
+        0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
 
     if (t->tm_year < 70) {
         return (time_t)-1;
     }
 
-    n = t->tm_year + 1900 - 1;
-    epoch = (t->tm_year - 70) * YEAR +
-            ((n / 4 - n / 100 + n / 400) - (1969 / 4 - 1969 / 100 + 1969 / 400)) * DAY;
-
-    y = t->tm_year + 1900;
-    m = 0;
-
-    for (i = 0; i < t->tm_mon; i++) {
-        epoch += mon [m] * DAY;
-        if (m == 1 && y % 4 == 0 && (y % 100 != 0 || y % 400 == 0)) {
-            epoch += DAY;
-        }
-
-        if (++m > 11) {
-            m = 0;
-            y++;
-        }
+    /*
+      every caller derives tm_mon from an external date field as
+      "month - 1", so it can land outside 0-11 when the source reports
+      a zero or out-of-range month.  Both cases are kept as the month
+      loop this replaced behaved: it walked its own month and year
+      counters forward, so months past December rolled into the
+      following years, and a negative month added nothing because the
+      loop simply never ran.
+     */
+    int mon = t->tm_mon;
+    int year_off = t->tm_year;
+    if (mon > 11) {
+        year_off += mon / 12;
+        mon %= 12;
+    } else if (mon < 0) {
+        mon = 0;
     }
 
-    epoch += (t->tm_mday - 1) * DAY;
-    epoch += t->tm_hour * HOUR + t->tm_min * MINUTE + t->tm_sec;
+    const uint32_t year = (uint32_t)year_off + 1900U;
+    const uint32_t n = year - 1U;
+    const bool leap = (year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0);
 
-    return epoch;
+    // leap days since the epoch, as a closed form rather than a
+    // per-year loop; 477 is the same count taken at 1969
+    const uint32_t days = (uint32_t)(year_off - 70) * 365U
+        + (n/4U - n/100U + n/400U) - 477U
+        + cumdays[mon] + (leap && mon > 1)
+        + (uint32_t)t->tm_mday - 1U;
+
+    return (time_t)days * 86400 + t->tm_hour*3600 + t->tm_min*60 + t->tm_sec;
 }
-