Due to the changes in timestamping items introduced in 4.0 it is no longer possible to track/trigger when an active agent client has been offline for X minutes if that client's clock is inaccurate.

For example, using agent.ping.nodata(5m) doesn't work very well when the client clock is 6 minutes behind the server and its clock is used to timestamp the incoming value.

In lieu of fixing the timestamps for all items for active agents I would suggest a new item like "host.lastcontact" that gets timestamped by the server every time there is communication with the host so we can properly detect offline clients.

Here is a forum thread discussing this issue in more detail:

https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/373803-zabbix-agent-ping-timestamp