It seem that when using calendars.setOnClickHandler(onDateChange); on double calendar, when a date is clicked on the left calendar, it comes in the handler as a Date object, and when it is clicked from the right calendar, it comes as a string object.
I had to do something like that :
function onDateChange(date, obj, type) {
if (type == ‘left’) {
alert ("Begin : " + date.getDate() + “/” + date.getMonth() + “/” + date.getYear());
} else {
date = new Date(date);
alert ("End : " + date.getDate() + “/” + date.getMonth() + “/” + date.getYear());
}
}