Yeah, I think RRDtool gets confused when the time jumps around. Depending on which way it changed, I think either the graphs will come back on their own, or you'll have to manually delete the files in /usr/local/nagios/share/perfdata/* and then they'll regenerate.
http://go.nagios.com/faq#Performance_graph_Problems
To clarify rrd uses GMT time, so adjusting the clock's time zone or for daylight savings in Linux will have no effect. Keep in mind that Windows will change the clock, keeping it correct for the current time zone and day of year.
rrd tool used to depend on receiving newer information. Adjusting physical time backwards will result in rrd tool dropping results until time once again starts moving forwards.
As for your problem there should have been no relation to the graphs and the currently configured time zone, please provide more information.