I’ll keep this somewhat short. 
I saw the update for Matter Server 9.1.0 last night. Including this gem in the release notes:-
Add time_sync option (auto/on/off, default auto) to push the current time to Matter devices via the Matter Server’s --enable-time-sync flag. auto enables time sync only when the host clock is NTP synchronized; on always enables it (with a warning if NTP is not synchronized); off disables it.
Huzzah! No more resetting my Ikea ALPSTUGA’s scattered around the house after every brown-out or kid pulling the power! No more having them all just a few minutes off from each other! I can have a clock I can trust without messing with it regularly!! Whooo!!!
But a warning to everyone else with them… It didn’t work. 
I checked my HA NTP. I checked the ALPSTUGA. I checked and restarted HA and the matter server… wasn’t working. Booo… Then I was reading about Matter updates when suddenly it DID WORK! 
Shortly after I found this gem in the matter.js changelog:-
Feature: Adds automatic time synchronization for devices that support it, pushing host time (UTC and time zone / DST). Enable it with the --enable-time-sync CLI option (env ENABLE_TIME_SYNC) when the host has a reliable, synced time source. Runs first 30-60 minutes after start, then every 24h.
Oh. Welp. That would have been good to know before I wasted a bunch of time trying to get it to work. Also, my one quick test (eg:- I’ve not put any significant time into this and admit I could have done something wrong), resetting one of the ALPSTUGA’s did NOT fix the time. It started over at 00:00. I’m letting it sit but I’m guessing that it won’t have the time synced until whenever that 24hr period elapses.
Personally, I wouldn’t mind a sync on a little faster schedule then that. Or at least whenever a device starts up for the first time.
But honestly? If even nothing else changes and it’s just this from now on, I am so very very very very very very very VERY pleased not to have to go around and reset all of the clocks.
Worst case scenario of something like a brown-out that resets them all (but doesn’t reset HA on battery backup), I know I can just restart the matter server integration and some random time between 30-60 minutes all of them will get the right time.
A big thanks of profound gratitude for all the Matter devs in getting this feature added! It’s a literal time saver for me to not have to reset the clocks 
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