feat: DS3231 RTC read/set with lost-power detection

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/WeatherPredictor.ino

@@ -3,6 +3,7 @@
 #include "config.h"
 #include "display_ui.h"
 #include "sensors.h"
+#include "rtc_time.h"
 
 static void i2cScan() {
   Serial.println(F("I2C scan:"));
@@ -28,12 +29,18 @@ void setup() {
   displaySplash("Weather", "Predictor v0.1");
 
   if (!sensorsBegin()) Serial.println(F("BMP180 not found!"));
+  if (!rtcBegin())     Serial.println(F("DS3231 not found!"));
+  if (rtcLostPower()) {
+    Serial.println(F("RTC lost power -> setting to build time"));
+    rtcSet(RtcTime{2026, 7, 17, 12, 0, 0});  // temporary; NTP will correct later
+  }
 }
 
 void loop() {
+  RtcTime t = rtcNow();
   float abs_ = readAbsPressureHpa();
-  float msl  = toSeaLevelHpa(abs_, DEFAULT_ALTITUDE_M);
-  Serial.printf("abs=%.1f hPa  msl=%.1f hPa  t=%.1f C\n",
-                abs_, msl, readTemperatureC());
+  Serial.printf("%04u-%02u-%02u %02u:%02u:%02u  abs=%.1f msl=%.1f t=%.1f\n",
+                t.year, t.month, t.day, t.hour, t.minute, t.second,
+                abs_, toSeaLevelHpa(abs_, DEFAULT_ALTITUDE_M), readTemperatureC());
   delay(2000);
 }

WeatherPredictor/rtc_time.cpp

@@ -0,0 +1,20 @@
+#include <RTClib.h>
+#include "rtc_time.h"
+
+static RTC_DS3231 rtc;
+
+bool rtcBegin()      { return rtc.begin(); }
+bool rtcLostPower()  { return rtc.lostPower(); }
+
+RtcTime rtcNow() {
+  DateTime n = rtc.now();
+  return RtcTime{ n.year(), n.month(), n.day(), n.hour(), n.minute(), n.second() };
+}
+
+void rtcSet(const RtcTime& t) {
+  rtc.adjust(DateTime(t.year, t.month, t.day, t.hour, t.minute, t.second));
+}
+
+uint32_t rtcEpoch() {
+  return rtc.now().unixtime();
+}

WeatherPredictor/rtc_time.h

@@ -0,0 +1,13 @@
+#pragma once
+#include <Arduino.h>
+
+struct RtcTime {
+  uint16_t year;
+  uint8_t  month, day, hour, minute, second;
+};
+
+bool     rtcBegin();
+bool     rtcLostPower();
+RtcTime  rtcNow();
+void     rtcSet(const RtcTime& t);
+uint32_t rtcEpoch();