swclock on Raspberry Pi

[anthumchris]

Could swclock save and preserve the time/clock after rebooting a battery-less Raspberry Pi (RPI)? I don't fully understand what swclock is meant to be used for and if that's possible.

Problem & testing

Some services incorrectly show as being started days ago. I'm running diskless (in-memory) Alpine Linux with busybox NTP (network time protocol), and the RPI time isn't preserved on reboots. These start times are calculated from the read-only Alpine ISO files:

$ rc-status -a | grep -E 'swclock|wpa_|go2rtc'
 wpa_supplicant                             [  started 121 day(s) 18:17:29 (0) ]
 wpa_cli                                    [  started 121 day(s) 18:17:24 (0) ]
 swclock                                                           [  started  ]
 go2rtc                                     [  started 121 day(s) 18:17:25 (0) ]

I modified /etc/conf.d/swclock to use a persistent file with an explicit timestamp from midnight yesterday:

$ grep -E '^swclock_file' /etc/conf.d/swclock
swclock_file=/media/mmcblk0p2/swclock.save

$ date
Thu Mar 19 09:00:48 MDT 2026

$ touch -t 202603180000 /media/mmcblk0p2/swclock.save    # timestamp as yesterday
$ ls --full-time /media/mmcblk0p2/swclock.save
-rw-r--r--    1 root     root             0 2026-03-18 00:00:00 -0600 /media/mmcblk0p2/swclock.save

After rebooting, the services now show as being started yesterday:

$ rc-status -a | grep -E 'wpa_|go2rtc'
 go2rtc                                       [  started 1 day(s) 09:06:18 (0) ]
 wpa_cli                                      [  started 1 day(s) 09:06:17 (0) ]
 swclock                                                           [  started  ]
 wpa_supplicant                               [  started 1 day(s) 09:06:22 (0) ]

The services don't show a newer start time until I manually restart swclock and reboot:

$ rc-service swclock restart
 * WARNING: you are stopping a boot service
 * WARNING: you are stopping a boot service
 * Stopping busybox syslog ...                                [ ok ]
 * Saving the shutdown time ...                               [ ok ]
 * Setting the local clock based on last shutdown time ...    [ ok ]
 * Starting busybox syslog ...                                [ ok ]

$ reboot

(after reboot):

$ rc-status -a | grep -E 'swclock|wpa_|go2rtc'
 swclock                                                           [  started  ]
 wpa_supplicant                                        [  started 00:02:24 (0) ]
 wpa_cli                                               [  started 00:02:19 (0) ]
 go2rtc                                                [  started 00:02:20 (0) ]