Fix timestamp calculations when RTC clock is corrected backwards

When the RTC drifts ahead and is corrected via clock sync, stored
timestamps can appear to be in the future, causing underflow in
"time ago" calculations (wrapping to ~4 billion seconds).

Changes:
- Add safeElapsedSecs() helper that clamps to 0 if timestamp > now
- Apply to neighbor "heard X ago" displays in simple_repeater
- Apply to UI time displays in companion_radio
- Apply to TimeSeriesData calculations in simple_sensor
- Switch BaseChatMesh connection expiry from RTC to millis()

The connection expiry change is the most important: using monotonic
time (millis) makes it immune to RTC adjustments from GPS, NTP, or
manual sync.

Author: Wessel Nieboer

examples/companion_radio/ui-new/UITask.cpp

@@ -225,7 +225,7 @@ class HomeScreen : public UIScreen {
       for (int i = 0; i < UI_RECENT_LIST_SIZE; i++, y += 11) {
         auto a = &recent[i];
         if (a->name[0] == 0) continue;  // empty slot
-        int secs = _rtc->getCurrentTime() - a->recv_timestamp;
+        uint32_t secs = safeElapsedSecs(_rtc->getCurrentTime(), a->recv_timestamp);
         if (secs < 60) {
           sprintf(tmp, "%ds", secs);
         } else if (secs < 60*60) {
@@ -488,7 +488,7 @@ class MsgPreviewScreen : public UIScreen {
 
     auto p = &unread[head];
 
-    int secs = _rtc->getCurrentTime() - p->timestamp;
+    uint32_t secs = safeElapsedSecs(_rtc->getCurrentTime(), p->timestamp);
     if (secs < 60) {
       sprintf(tmp, "%ds", secs);
     } else if (secs < 60*60) {

examples/simple_repeater/MyMesh.cpp

@@ -343,7 +343,7 @@ int MyMesh::handleRequest(ClientInfo *sender, uint32_t sender_timestamp, uint8_t
 
         // add next neighbour to results
         auto neighbour = sorted_neighbours[index + offset];
-        uint32_t heard_seconds_ago = getRTCClock()->getCurrentTime() - neighbour->heard_timestamp;
+        uint32_t heard_seconds_ago = safeElapsedSecs(getRTCClock()->getCurrentTime(), neighbour->heard_timestamp);
         memcpy(&results_buffer[results_offset], neighbour->id.pub_key, pubkey_prefix_length); results_offset += pubkey_prefix_length;
         memcpy(&results_buffer[results_offset], &heard_seconds_ago, 4); results_offset += 4;
         memcpy(&results_buffer[results_offset], &neighbour->snr, 1); results_offset += 1;
@@ -934,7 +934,7 @@ void MyMesh::formatNeighborsReply(char *reply) {
     mesh::Utils::toHex(hex, neighbour->id.pub_key, 4);
 
     // add next neighbour
-    uint32_t secs_ago = getRTCClock()->getCurrentTime() - neighbour->heard_timestamp;
+    uint32_t secs_ago = safeElapsedSecs(getRTCClock()->getCurrentTime(), neighbour->heard_timestamp);
     sprintf(dp, "%s:%d:%d", hex, secs_ago, neighbour->snr);
     while (*dp)
       dp++; // find end of string

examples/simple_sensor/TimeSeriesData.cpp

@@ -1,4 +1,5 @@
 #include "TimeSeriesData.h"
+#include <helpers/ArduinoHelpers.h>
 
 void TimeSeriesData::recordData(mesh::RTCClock* clock, float value) {
   uint32_t now = clock->getCurrentTime();
@@ -12,7 +13,7 @@ void TimeSeriesData::recordData(mesh::RTCClock* clock, float value) {
 
 void TimeSeriesData::calcMinMaxAvg(mesh::RTCClock* clock, uint32_t start_secs_ago, uint32_t end_secs_ago, MinMaxAvg* dest, uint8_t channel, uint8_t lpp_type) const {
   int i = next, n = num_slots;
-  uint32_t ago = clock->getCurrentTime() - last_timestamp;
+  uint32_t ago = safeElapsedSecs(clock->getCurrentTime(), last_timestamp);
   int num_values = 0;
   float total = 0.0f;
 

src/helpers/ArduinoHelpers.h

@@ -3,6 +3,15 @@
 #include <Mesh.h>
 #include <Arduino.h>
 
+// Safe elapsed time calculation that handles clock corrections (when RTC is set backwards).
+// Returns 0 if recorded_timestamp is in the "future" relative to current_time.
+inline uint32_t safeElapsedSecs(uint32_t current_time, uint32_t recorded_timestamp) {
+  if (recorded_timestamp > current_time) {
+    return 0;  // Clock was corrected backwards; treat as "just now"
+  }
+  return current_time - recorded_timestamp;
+}
+
 class VolatileRTCClock : public mesh::RTCClock {
   uint32_t base_time;
   uint64_t accumulator;

src/helpers/BaseChatMesh.cpp

@@ -609,7 +609,7 @@ bool BaseChatMesh::startConnection(const ContactInfo& contact, uint16_t keep_ali
   uint32_t interval = connections[use_idx].keep_alive_millis = ((uint32_t)keep_alive_secs)*1000;
   connections[use_idx].next_ping = futureMillis(interval);
   connections[use_idx].expected_ack = 0;
-  connections[use_idx].last_activity = getRTCClock()->getCurrentTime();
+  connections[use_idx].last_activity = _ms->getMillis();  // use monotonic time for connection expiry
   return true;  // success
 }
 
@@ -635,7 +635,7 @@ bool BaseChatMesh::hasConnectionTo(const uint8_t* pub_key) {
 void BaseChatMesh::markConnectionActive(const ContactInfo& contact) {
   for (int i = 0; i < MAX_CONNECTIONS; i++) {
     if (connections[i].keep_alive_millis > 0 && connections[i].server_id.matches(contact.id)) {
-      connections[i].last_activity = getRTCClock()->getCurrentTime();
+      connections[i].last_activity = _ms->getMillis();  // use monotonic time for connection expiry
 
       // re-schedule next KEEP_ALIVE, now that we have heard from server
       connections[i].next_ping = futureMillis(connections[i].keep_alive_millis);
@@ -649,7 +649,7 @@ ContactInfo* BaseChatMesh::checkConnectionsAck(const uint8_t* data) {
     if (connections[i].keep_alive_millis > 0 && memcmp(&connections[i].expected_ack, data, 4) == 0) {
       // yes, got an ack for our keep_alive request!
       connections[i].expected_ack = 0;
-      connections[i].last_activity = getRTCClock()->getCurrentTime();
+      connections[i].last_activity = _ms->getMillis();  // use monotonic time for connection expiry
 
       // re-schedule next KEEP_ALIVE, now that we have heard from server
       connections[i].next_ping = futureMillis(connections[i].keep_alive_millis);
@@ -666,9 +666,13 @@ void BaseChatMesh::checkConnections() {
   for (int i = 0; i < MAX_CONNECTIONS; i++) {
     if (connections[i].keep_alive_millis == 0) continue;  // unused slot
 
-    uint32_t now = getRTCClock()->getCurrentTime();
-    uint32_t expire_secs = (connections[i].keep_alive_millis / 1000) * 5 / 2;   // 2.5 x keep_alive interval
-    if (now >= connections[i].last_activity + expire_secs) {
+    // Use monotonic time (millis) for connection expiry - immune to RTC clock changes.
+    // Note: This assumes light sleep mode where millis() continues to increment.
+    // Deep sleep would reset millis(), but BaseChatMesh is only used by companion_radio
+    // which uses light sleep.
+    unsigned long now = _ms->getMillis();
+    uint32_t expire_millis = (connections[i].keep_alive_millis * 5) / 2;   // 2.5 x keep_alive interval
+    if ((now - connections[i].last_activity) >= expire_millis) {
       // connection now lost
       connections[i].keep_alive_millis = 0;
       connections[i].next_ping = 0;