Pin NTP to NICT, set Tokyo timezone; add time sync report

timesyncd now uses ntp.nict.jp (stratum 1), with the JP pool and Ubuntu as fallback, over whichever uplink holds the default route. Timezone moves from Asia/Yakutsk to Asia/Tokyo: same offset, right place. Records that the RTC loses time at power-off (the cold boot came up at June 4 until NTP synced), and the planned chrony + gpsd setup once GNSS has a fix. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
2026-10-08 09:24:33 +09:00
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+---
+type: report
+date: 2026-10-08
+---
+# Time Sync and Timezone
+
+## Result
+
+| Item | State |
+|------|-------|
+| Network time | `systemd-timesyncd`, primary `ntp.nict.jp` (stratum 1), synced over whichever uplink holds the default route (Ethernet / WiFi / LTE) |
+| Timezone | `Asia/Tokyo` (was `Asia/Yakutsk`: same +09:00 and no DST either, but the wrong place) |
+| Clock across power-off | **Not kept.** The RTC has no backup battery (see below) |
+| GPS time | Not yet. Waiting on a GNSS fix (see [SIM7600 report](SIM7600-SETUP.md)) |
+
+## RTC loses time when powered off
+
+On the 2026-10-08 cold boot, the kernel set the system clock from `rtc0` (`nvvrs-pseq-rtc`, `hctosys=1`) to **June 4**.
+Then the clock was corrected in two steps:
+
+1. ~12 s: timesyncd stepped forward to its last saved timestamp (`/var/lib/systemd/timesync/clock`). This is close, but behind by however long the board was off.
+2. ~43 s: first NTP sync corrected the rest. The kernel then writes the synced time back to the RTC, which only lasts until the next power-off.
+
+Until step 2, timestamps are wrong, and with no uplink (in the car, out of coverage) they stay wrong. The fix is hardware:
+an RTC backup battery (see [BOM](../../BOM.md#rtc-backup-battery)). `rtc1` (`tegra_rtc`) isn't used for boot time and reads 1970.
+
+## Configuration
+
+```
+sudo timedatectl set-timezone Asia/Tokyo
+```
+
+`/etc/systemd/timesyncd.conf.d/10-servers.conf`:
+
+```
+# Japan-local primary (NICT, national time standard), global pool as backup.
+[Time]
+NTP=ntp.nict.jp
+FallbackNTP=ntp.ubuntu.com 0.jp.pool.ntp.org 1.jp.pool.ntp.org 2.jp.pool.ntp.org
+```
+
+`sudo systemctl restart systemd-timesyncd`, then check with `timedatectl timesync-status` (server, stratum, offset).
+
+## Later: GPS as a time source
+
+timesyncd only speaks NTP. Once GNSS has a fix, replace it with `chrony`, configured with:
+
+- the same NTP servers as above, which remain the default whenever any uplink exists, and
+- a gpsd refclock (`refclock SHM 0`), which takes over when offline.
+
+NMEA over the USB serial port carries no PPS signal, so GPS time alone is only good to tens of milliseconds. That is plenty for log timestamps.