Solved by changing the way I was getting the "today" Date, using a var instead of using now, which is predefined:

Code:
var today:Date = new Date();
form.chegada_mc.selectableRange = {rangeStart:new Date(today.getFullYear(), today.getMonth(), today.getDate())};
form.saida_mc.selectableRange = {rangeStart:new Date(today.getFullYear(), today.getMonth(), today.getDate()+1)};
Thanks anyway.