LXPanel's clock default calendar has the WM_NAME "lxpanel". This makes it difficult to set window manager positioning rules (for example, Fluxbox).
Can you please give the calendar a unique WM_NAME?
I'm not fully sure if this is connected to the issue reported here but on dual monitor setup when lxpanel is on left/main monitor (panel set to 1023px wide on 1024x768 screen) and 'digital clock' is psitioned on panel's right edge then the calendar will appear one half of it on the left monitor and other half on the other, forcing user to drag it to the correct/usable position every time.
Setting different WM_NAME for windows in the same application is forbidden by specification and is usually impossible. Although there was incorrect window type hint set for the calendar. I believe this issue is completely fixed in latest version of LXPanel - 0.7.0. Try it, please, and let us know if something is still wrong. Thank you very much.