J'ai trouvé out the same problem in my horizontal Taskbar. Il s'avère que that when "Use small taskbar buttons" is checked, la barre des tâches height is not enough to house the time and the date strings, so the date string is dropped.
Unchecking "Use small taskbar buttons" or doubling la barre des tâches height are the two solutions J'ai trouvé... but they neutralize my initial intention of a smaller taskbar taking up less space.
[EDIT]
After reading Why vous devriez use a vertical taskbar J'ai essayé it and also realized that you get the full time and date already from the smallest size of the vertical taskbar, even with small taskbar buttons. So it might have changed since the time the question was asked. Now, this might be your solution if you like the side bar, or you are willing to try and reap some of the benefits that article claims.
