So it looks like the AS time does not work well when the router has been up for weeks, or that's my guess.
I could be wrong, but I don't think it's to do with router uptime. I think that, with some routers such as the DSL-2740B, the AS value always gives the time since the
first connection after a reboot. So if you get a re-sync while the router remains running, the AS reference time doesn't change.
Have you any ideas as to why this might be? Are there any other rs-ux users with an HG612 on ADSL trying this out?
Yes, I use an HG612. You need to change the "Stats command prefix" to "xdslcmd info" for this router, so perhaps you haven't done that.