Apply local UTC offset in Pebble RTC

Add pbl_rtc_tz_offset() to return a cached local-time offset (seconds).
It honors the TZ_OFFSET_SEC environment variable and falls back to the
host's localtime detection. pbl_rtc_get_time() now returns UTC plus
that offset so the firmware sees a local wall-clock unix timestamp.

Author: jplexer

pebble/hw/misc/pebble_rtc.c

@@ -21,6 +21,9 @@
 #include "hw/irq.h"
 #include "hw/sysbus.h"
 
+#include <time.h>
+#include <stdlib.h>
+
 #define TYPE_PEBBLE_GENERIC_RTC "pebble-rtc"
 OBJECT_DECLARE_SIMPLE_TYPE(PblRtc, PEBBLE_GENERIC_RTC)
 
@@ -56,9 +59,41 @@ static void pbl_rtc_update_irq(PblRtc *s)
                           (s->ctrl & CTRL_ALARM_IRQ));
 }
 
+/* Return the local-time UTC offset in seconds, cached on first call.
+ *
+ * Honors the TZ_OFFSET_SEC environment variable (seconds east of UTC) when set
+ * — this matches the stm32 Pebble RTC and is how the browser/JS build injects
+ * the user's timezone. When unset, falls back to the host's detected local
+ * offset via localtime_r(). */
+static int64_t pbl_rtc_tz_offset(void)
+{
+    static int64_t offset_sec;
+    static bool cached;
+
+    if (!cached) {
+        const char *env = getenv("TZ_OFFSET_SEC");
+        if (env) {
+            offset_sec = (int64_t)atoll(env);
+        } else {
+            time_t now = time(NULL);
+            struct tm lt;
+            if (localtime_r(&now, &lt)) {
+                offset_sec = (int64_t)lt.tm_gmtoff;
+            }
+        }
+        cached = true;
+    }
+    return offset_sec;
+}
+
+/* Return "local time as a unix timestamp". Pebble firmware stores wall-clock
+ * time in this form, so the value read here is shifted by the local UTC
+ * offset — a guest that applies no further timezone conversion will still
+ * display the user's local wall-clock time. */
 static uint64_t pbl_rtc_get_time(void)
 {
-    return (uint64_t)qemu_clock_get_ms(QEMU_CLOCK_HOST) / 1000;
+    uint64_t utc = (uint64_t)qemu_clock_get_ms(QEMU_CLOCK_HOST) / 1000;
+    return utc + pbl_rtc_tz_offset();
 }
 
 static uint64_t pbl_rtc_read(void *opaque, hwaddr offset, unsigned size)