There are times when it would be useful to trigger on the compare the timestamps of two log entries.
For a real world example here HP Proliant Serves will send a one trap when a fan or power supply fails and a different trap when they are OK. Using logtime one would set a trigger when the timestamp of the first trap is greater then the second one.
This trigger would be defined as:
{Template SNMP Traps HP Proliant Fans:snmptrap[.1.3.6.1.4.1.232.0.6037].logtime()} > {Template SNMP Traps HP Proliant Fans:snmptrap[.1.3.6.1.4.1.232.0.6055].logtime()}

This would be just one example of one such use.
An other would be to check the timestamp against the current time, or a static number, or any item that returns a number

Attached are a couple of patches. One against zbxserver evalfunc.c and the others fix frontend website. Patches were made with 3.0.1