It doesn't happen always, but quite often (around 75% of all cases). Newly added clock applet doesn't bring up tooltip window when I move the mouse over it, hence it doesn't show the date. After restarting the panel or the whole Xfce, it is okay.
Ok, I've change a small thing in the module, but I really can't reproduce this. Please reopen if it still happens with the upcoming 4.2.1 release. Thanks.