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--- comfilepi:set_date_and_time [2026/03/27 00:35] – [Drift Compensation] mfranklin
+++ comfilepi:set_date_and_time [2026/04/05 23:54] (current) – [System Clock Compensation] mfranklin
@@ Line 14: / Line 14: @@
 ===== Drift Compensation =====
+There are two clocks that have the potential to drift:  The RTC and the operating system's system clock.  Both will drift independently, so the Linux operating system provides utilities to configure compensation for both.
+==== RTC Drift Compensation ====
 If you notice the RTC drifting over time, and the drift is consistent and predictable, it is possible to configure the OS to compensate for the drift using the following procedure:
-  - With Internet time synchronization disabled, run <html><code>sudo ntpdate pool.ntp.org</code></html> to manually synchronize the system time to an Internet time server.
+  - With Internet time synchronization disabled, but with an Internet connection, run <html><code>sudo ntpdate pool.ntp.org</code></html> to manually synchronize the system time to an Internet time server.
   - Run <html><code>sudo hwclock --systohc --utc --update-drift</code></html> to synchronize the RTC with the system time, and to ensure the ''/etc/adjtime'' records the last calibration time.
   - Power off the panel PC for a few days; the longer the better.
@@ Line 25: / Line 30: @@
 That procedure will update the [[https://manpages.debian.org/trixie/util-linux/adjtime_config.5.en.html|/etc/adjtime]] file with a drift compensation measurement determined from the above procedure.
+Finally, to ensure the system time is updated from the RTC, with drift compensation, create the following udev rule file at ///etc/udev/rules.d/85-hwclock.rules//:
+<code>
+ACTION=="add", SUBSYSTEM=="rtc", KERNEL=="rtc0", TAG+="systemd", ENV{SYSTEMD_WANTS}+="hwclock.service"
+</code>
+And create the following service at ///etc/systemd/system/hwclock.service//:
+<code>
+[Unit]
+Description=Set system clock from RTC with adjtime compensation
+ConditionPathExists=/dev/rtc0
+ConditionPathExists=/etc/adjtime
+[Service]
+Type=oneshot
+ExecCondition=/bin/sh -ec 'timedatectl show --property=NTP 2>/dev/null | grep -qx "NTP=no"'
+ExecStart=/sbin/hwclock --rtc=/dev/rtc0 --hctosys
+</code>
+Reboot, and after booting, the ComfilePi should have its system time synchronized with the RTC, and adjusted to compensate for the RTC's drift.  Confirm with the ''date'' command.
+==== System Clock Compensation ====
+After configuring the RTC's drift compensation, the system clock's drift compensation can be configured.  For that we need [[https://manpages.debian.org/trixie/adjtimex/adjtimexconfig.8.en.html|adjtimexconfig]].
+Run the following commands in a terminal.
+<code>
+sudo apt update
+sudo apt install adjtimex
+sudo adjtimexconfig
+sudo systemctl enable adjtimex.service
+</code>
+The ''adjustimexconfig'' command will measure the amount of drift the system clock has relative to the RTC, save it to ///etc/default/adjtimex// and the //adjtimex.service// will load that at every boot to compensate.
 [[comfilepi:index|ComfilePi - Industrial Raspberry Pi Panel PC]]

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