Zabbix uses received value for clock to calculate "delete date" for not discovered items (LLD).
The current formula is "Received clock + Keep lost resources period (in days)".
It is better to use formula like "Now + Keep lost resources period (in days)" to avoid any issues. Example, incorrect clock may cause the following:
"The item is not discovered anymore and will be deleted in 19y 11m 7d (on 2035-10-17 at 12:27)".