NTP monitoring: Comparing SNMP-based time offset with Zabbix system time

[kyus]

You can create an item in the "Zabbix server" host that retrieves system.localtime

[HideOnBush]

There is an item in the Zabbix server; how do I "connect" the two different items on two different hosts with a trigger?

[kyus]

Like: last(/NTP/ntp.convert)-last(/ZabbixServer/system.localtime)>15

Basically like this. You'll notice that the trigger will appear in both hosts (only 1 event will be generated in case of problem).

[HideOnBush]

Basically like this. You'll notice that the trigger will appear in both hosts (only 1 event will be generated in case of problem).

Ever the same problem, I made the trigger with two different hosts but is not collecting data

[kyus]

Well this is a problem with your items, not the trigger.
You need to find out why your items can't collect data, it can be either ntp.convert or system.localtime that aren't working as expected