feat(mqtt): enhance timestamp formatting with microsecond precision

Updated the MQTTMessageBuilder to include microsecond precision in the
timestamp formatting. The formatIsoTimestampForMqtt function now accepts
a microsecond parameter, allowing for more accurate time representation.
Modified related functions in MQTTBridge to utilize the new timestamp
formatting, ensuring consistency across MQTT messages.

src/helpers/MQTTMessageBuilder.cpp

@@ -3,18 +3,26 @@
 #include <cstring>
 #include <math.h>
 #include <time.h>
+#include <sys/time.h>
 #include <Timezone.h>
 #include "MeshCore.h"
 
-void MQTTMessageBuilder::formatIsoTimestampForMqtt(time_t now, Timezone* timezone, char* buffer, size_t buffer_size) {
+void MQTTMessageBuilder::formatIsoTimestampForMqtt(time_t now, long usec, Timezone* timezone, char* buffer, size_t buffer_size) {
   if (!buffer || buffer_size == 0) return;
   // Always emit UTC with an explicit "+00:00" offset, matching Python's
   // datetime.now(timezone.utc).isoformat(). The system clock is UTC (SNTP offset 0),
   // so gmtime() is correct regardless of the prefs Timezone (now unused here).
   (void)timezone;
+  // Clamp the sub-second to a valid microsecond range so the "%06ld" field can never
+  // overflow to 7 digits or go negative on a bad clock read.
+  if (usec < 0) usec = 0;
+  else if (usec > 999999) usec = 999999;
   struct tm* tm_info = gmtime(&now);
-  if (tm_info && strftime(buffer, buffer_size, "%Y-%m-%dT%H:%M:%S.000000+00:00", tm_info) > 0) {
-    return;
+  if (tm_info) {
+    size_t n = strftime(buffer, buffer_size, "%Y-%m-%dT%H:%M:%S", tm_info);
+    if (n > 0 && snprintf(buffer + n, buffer_size - n, ".%06ld+00:00", usec) > 0) {
+      return;
+    }
   }
   strncpy(buffer, "2024-01-01T12:00:00.000000+00:00", buffer_size - 1);
   buffer[buffer_size - 1] = '\0';
@@ -231,9 +239,13 @@ int MQTTMessageBuilder::buildPacketJSON(
 ) {
   if (!packet) return 0;
   
-  time_t now = time(nullptr);
+  // One wall-clock read: tv_sec feeds both the timestamp and the UTC time/date
+  // fields below (kept consistent), tv_usec is the real sub-second.
+  struct timeval now_tv;
+  gettimeofday(&now_tv, nullptr);
+  time_t now = now_tv.tv_sec;
   char timestamp[40];
-  formatIsoTimestampForMqtt(now, timezone, timestamp, sizeof(timestamp));
+  formatIsoTimestampForMqtt(now, now_tv.tv_usec, timezone, timestamp, sizeof(timestamp));
   
   // Packet time/date: UTC (gmtime), same family as meshcoretomqtt serial fields
   struct tm* utc_timeinfo = gmtime(&now);
@@ -305,9 +317,13 @@ int MQTTMessageBuilder::buildPacketJSONFromRaw(
 ) {
   if (!packet || !raw_data || raw_len <= 0) return 0;
   
-  time_t now = time(nullptr);
+  // One wall-clock read: tv_sec feeds both the timestamp and the UTC time/date
+  // fields below (kept consistent), tv_usec is the real sub-second.
+  struct timeval now_tv;
+  gettimeofday(&now_tv, nullptr);
+  time_t now = now_tv.tv_sec;
   char timestamp[40];
-  formatIsoTimestampForMqtt(now, timezone, timestamp, sizeof(timestamp));
+  formatIsoTimestampForMqtt(now, now_tv.tv_usec, timezone, timestamp, sizeof(timestamp));
   
   struct tm* utc_timeinfo = gmtime(&now);
   
@@ -370,11 +386,13 @@ int MQTTMessageBuilder::buildRawJSON(
   size_t buffer_size
 ) {
   if (!packet) return 0;
-  
-  time_t now = time(nullptr);
+
+  // One wall-clock read: tv_sec for the timestamp, tv_usec for the real sub-second.
+  struct timeval now_tv;
+  gettimeofday(&now_tv, nullptr);
   char timestamp[40];
-  formatIsoTimestampForMqtt(now, timezone, timestamp, sizeof(timestamp));
-  
+  formatIsoTimestampForMqtt(now_tv.tv_sec, now_tv.tv_usec, timezone, timestamp, sizeof(timestamp));
+
   // Convert packet to hex
   // MAX_TRANS_UNIT is 255, so max hex size is 510 chars + null = 511 bytes
   char raw_hex[1024];

src/helpers/MQTTMessageBuilder.h

@@ -26,11 +26,16 @@ public:
   /**
    * Format the MQTT JSON `timestamp` field (same rule for status, packet, raw).
    * Always UTC with an explicit "+00:00" offset, ISO-8601
-   * "%Y-%m-%dT%H:%M:%S.000000+00:00" (matches Python
+   * "%Y-%m-%dT%H:%M:%S.uuuuuu+00:00" (matches Python
    * datetime.now(timezone.utc).isoformat()). The `timezone` parameter is retained
    * for API compatibility but ignored — the system clock is UTC.
+   *
+   * `usec` is the sub-second component in microseconds (0..999999), normally taken
+   * from the same gettimeofday() read as `now` so the two don't tear at a second
+   * boundary. It is a real sub-second (SNTP-maintained wall clock), not a literal;
+   * pass 0 if no sub-second source is available.
    */
-  static void formatIsoTimestampForMqtt(time_t now, Timezone* timezone, char* buffer, size_t buffer_size);
+  static void formatIsoTimestampForMqtt(time_t now, long usec, Timezone* timezone, char* buffer, size_t buffer_size);
 
   /**
    * Build status message JSON

src/helpers/bridges/MQTTBridge.cpp

@@ -5,6 +5,7 @@
 #include <WiFiUdp.h>
 #include <Timezone.h>
 #include <time.h>
+#include <sys/time.h>
 #include <math.h>
 
 #ifdef WITH_SNMP
@@ -1712,7 +1713,9 @@ void MQTTBridge::publishStatusToSlot(int index) {
 
   // Status timestamp: UTC with explicit +00:00 offset, same as packet/raw JSON
   // `timestamp` (system clock is UTC — SNTP offset 0; prefs Timezone is separate).
-  MQTTMessageBuilder::formatIsoTimestampForMqtt(time(nullptr), _timezone, timestamp, sizeof(timestamp));
+  struct timeval now_tv;
+  gettimeofday(&now_tv, nullptr);
+  MQTTMessageBuilder::formatIsoTimestampForMqtt(now_tv.tv_sec, now_tv.tv_usec, _timezone, timestamp, sizeof(timestamp));
 
   snprintf(radio_info, sizeof(radio_info), "%.6f,%.1f,%d,%d",
            _prefs->freq, _prefs->bw, _prefs->sf, _prefs->cr);
@@ -2423,7 +2426,9 @@ bool MQTTBridge::publishStatus() {
 
   // Status timestamp: UTC with explicit +00:00 offset, same as packet/raw JSON
   // `timestamp` (system clock is UTC — SNTP offset 0; prefs Timezone is separate).
-  MQTTMessageBuilder::formatIsoTimestampForMqtt(time(nullptr), _timezone, timestamp, sizeof(timestamp));
+  struct timeval now_tv;
+  gettimeofday(&now_tv, nullptr);
+  MQTTMessageBuilder::formatIsoTimestampForMqtt(now_tv.tv_sec, now_tv.tv_usec, _timezone, timestamp, sizeof(timestamp));
 
   snprintf(radio_info, sizeof(radio_info), "%.6f,%.1f,%d,%d",
            _prefs->freq, _prefs->bw, _prefs->sf, _prefs->cr);