diff --git a/WeatherPredictor/history.cpp b/WeatherPredictor/history.cpp
index 0df389d..c83dafb 100644
--- a/WeatherPredictor/history.cpp
+++ b/WeatherPredictor/history.cpp
@@ -26,10 +26,15 @@ Sample historyLatest() { return historyGet(s_count - 1); }
 bool historyTrendDelta(float& outDelta) {
   if (s_count < 2) return false;
   Sample latest = historyLatest();
+  if (latest.epoch < 10800UL) return false;
   uint32_t target = latest.epoch - 10800UL;   // 3 h earlier
+  // Pick the newest sample at or before `target`. Scan all samples (do not
+  // assume epochs are strictly increasing: an NTP correction can shift them).
   int idx = -1;
+  uint32_t bestEpoch = 0;
   for (int i = 0; i < s_count; i++) {
-    if (historyGet(i).epoch <= target) idx = i; else break;
+    Sample s = historyGet(i);
+    if (s.epoch <= target && s.epoch >= bestEpoch) { bestEpoch = s.epoch; idx = i; }
   }
   if (idx < 0) return false;
   Sample past = historyGet(idx);