Steps to reproduce:

Add devices with long uptime longer than 497 days to monitoring

Devices with up time longer than 998 days

I assume that this is some sort of overflow issue, as ALL hosts will exhibit the same exact behavior right around the same uptime - a little bit before 498  days (seems to be about 497) and again after the same period 998 days. 

Result:
I end up seeing a false reboot event.

Keep monitoring at 998 days again false reboot event. 

This causes havoc monitoring core equipment. This issue was here since version 4 of Zabbix that i am aware.