I tried what you say, but it didn't work... that's because in skin.xml you write: window="Digital Clock" and elements.xml isn't Digital Clock.
But, if you directly set:
<variable name="monthSize" default="28" description="Taille mois" />
It will work, but the variable monthSize will be global. If you have another window and you want a variable also named monthSize there will be a collision. But for only one window, there's no problem.
Maybe Rainy wanna make another kind of variable?