Steps to reproduce:
1. Goto Monitoring -> Screens
2. See "(now!)" in period string (eg. "21.01.2010 12:16 - 21.01.2010 13:16 (now!)"), graphs contains data for last hour
3. Wait a few minutes, eg.2
4. Select a different screen
5. See no "(new!)" in period string , graphs contains data for the same period as in previous screen, ""21.01.2010 12:16 - 21.01.2010 13:16", but we have already 13:18 now.

I think, until user doesn't specify directly what period he wants, especially starttime, system should assume endtime=now to display current (fresh) data.