It seems there's a problem with the tooltip in this example http://dev.sencha.com/deploy/dev/examples/form/adv-vtypes.html
If you change a couple of times the selected date in one of the two date fields and you try to move the mouse on the gray dates the tooltip says something like "The date in this field must be equals to or after SOMETHING" where SOMETHING is the first date you set and not the last one as it should be.