I remembered seeing some command which could synchronize the system clock to the digital TV (DVB) signal. This is ideal for a MythTV box which isn't connected to the internet.  It's dvbdate. One problem: it ignores the system time zone. It can use the TZ environment variable, whose current value can be obtained using the date command:

date +'%z'

so this will set the date:

sudo TZ=$(date +'%:z') dvbdate --set

so you can put this script in /etc/cron.hourly, and make it executable:

#!/bin/sh -e
TZ=$(date +'%:z') dvbdate --set