run the fiddle with zk 9.0.0
click the button on the top right go to next Month.
click a date which number less then today.(e.g. today is 4/9 than click 5/1~5/8)
click the button on the top left go back to previous Month.
also try select past date by keyboard, it should not be selectable.
the current value(selected date) did not respect the constraint(no past)
the current value should respect constraint
maybe check constraint before the change, if the change will break the constraint then don't do it.