This has become a lot more obvious now that systemd no longer writes out the system clock to the hardware clock on shutdown (which was a bad idea).

The ntp daemon is supposed to either enable 11 minute mode (kernel syncs the clock) or update the clock itself. It's better for the ntp daemon to do the updates itself because the kernel is quite naive in regards to drift and only syncs minutes/seconds.

Chrony can do both, but the default configuration file that Arch is using does neither. The rtcfile line should be uncommented in order to have chrony manage the clock properly.

This isn't done upstream because not all kernels are compiled with the ability to let chrony manage it, so I think it would be appropriate to do it in the package by default (since the Arch kernel does have support).

Without syncing the RTC clock, it will get more and more out-of-date and it will always be the time that the system starts with. Chrony will very slowly skew it to the right time, but it will take an incredible amount of time (days? weeks?) to correct a large difference like 10 hours.