I am posting this mainly for the sake of reference and, maybe, helping others with the same problem.
If, like us, you're running the Zabbix monitoring platform in Zulu time (aka UTC), you should have noticed a time glitch when displaying historical graphs.
Though we are strict in running all our servers in Zulu time, we haven't crossed the chasm and decided to run all our workstations and the rest of our life in UTC. So here is the simple fix you can apply to js/sbinit.js
The idea is to simply add the local browser time offset to the Unix time. With this fix in place, you will enjoy good looking graphs and correct navigation in them.
Time is really the stumbling block of software engineering...