smaller rtc fixes

src/rtc_ds3231.cpp

@@ -13,6 +13,23 @@ static uint8_t dec_to_bcd(uint8_t val) {
   return static_cast<uint8_t>(((val / 10) << 4) | (val % 10));
 }
 
+static time_t timegm_fallback(struct tm *tm_utc) {
+  if (!tm_utc) {
+    return static_cast<time_t>(-1);
+  }
+  char *old_tz = getenv("TZ");
+  setenv("TZ", "UTC0", 1);
+  tzset();
+  time_t t = mktime(tm_utc);
+  if (old_tz) {
+    setenv("TZ", old_tz, 1);
+  } else {
+    unsetenv("TZ");
+  }
+  tzset();
+  return t;
+}
+
 static bool read_registers(uint8_t start_reg, uint8_t *out, size_t len) {
   if (!out || len == 0) {
     return false;
@@ -77,7 +94,7 @@ bool rtc_ds3231_read_epoch(uint32_t &epoch_utc) {
   tm_utc.tm_year = year - 1900;
   tm_utc.tm_isdst = 0;
 
-  time_t t = timegm(&tm_utc);
+  time_t t = timegm_fallback(&tm_utc);
   if (t <= 0) {
     return false;
   }