why not use os system time

[jim-ling]

Erlang System Time may or may not align with os system time. My server was down for half a day due to a power outage.After restarting, some history messages was lost because the timestamp was incorrect. I noticed that the mod_mam used erlang:system_time.After that, our project changed elang:system_time to os:system_time and ran smoothly for half a year without any problems.

[weiss]

why not use os system time

Precisely to avoid issues with incorrect/jumping OS System Time. Erlang System Time allows for maintaining a uniform time across all nodes in case of clustering, and gives the VM control over how to cope with OS time warps.

After restarting, some history messages was lost because the timestamp was incorrect.

I can imagine clients failing to sync MAM correctly if new messages have an older timestamp than older messages (as opposed to actually loosing messages from the DB). If that's what happened, using OS System Time won't protect against that scenario.

In the end, you really want to configure your systems to have time synced before your daemons start up to avoid running into such issues, not only with ejabberd. There's no robust magic applications could apply to cope with OS time warps.