See http://www.zabbix.com/forum/showthread.php?t=12331 for full details.

In summary, if I select a screen in Monitoring/Dashboard/Screens then the time window (e.g. 1 hour, 7d or some other) is displayed from the current time back that amount.

If I select a slide-show from the same window, then the time period displayed is from when I first display the slide show. It never gets updated for the current time.