If there's a difference between the timezone of the Zabbix server and the client computer accessing the web interface the graph page will have the current time in the server tz (in the actual graph) and in the local tz (the html page, range selector).

Please have a look at the attached screenshot, I marked the 2 places with different times. The server is GMT + 3 and my computer is GMT + 2, so there's a 1h difference between the server and the computer I use to access the Zabbix web frontend. The time on the server is 15:49 and this is the right-most time on the graph, but the time on my computer is 14:49.

It's not a big deal if you just want to have a look at the latest graph, but if you want to go back in time for a certain period, select a graph interval everything gets messed up.

In my opinion both times should be in the server timezone.