Open a graph, check an URL for generated png and for "stime" parameter notice correct value like 20151025140523.
Click on "right" arrow in the time slider, notice how the stime gets a weird value like 20171024140524.
The same happens if you drag and drop a slider on right to the arrow or when try to expand the slider to right on the arrow.
Another way to reproduce: one a graph, click All to expand the slider to full available period, refresh the page, click left arrow or expand time slider on left.
Looks like it doesn't affect the png graph rendering in any bad way, but that still not very good as it may mislead zabbix users, which just has happened with me.
When try to display different graph periods, difference between good and bad values is a bit "similar", for example:
Reproduced on all versions listed in Affects Version/s, using Chrome and Opera under Linux.