Rainlendar should check from time to time whether he is up to date with the system time and refresh the tray icon.
This is already done once per second.
At least the "refresh" option in the menu should update the try icon as well.
Refresh will remove the tray icon and recreates it again.
Anyway this is a known problem
. In most systems the tray icon gets updated correctly but there seems to be some Windows installations where this doesn't happen. Unfortunately I have not found any such system where the problem exists so I have not been able to debug the problem.