I was finally able, with Behaviors, to do what I wanted: to copy the selected date from one calendar to the other...

So, let's say I have a start date field and an end date field. When I choose a date from the start date, I can set the date to be the minimum date of the end date field. Great!

BUT, there are two problems:
1- it doesn't work if I select the onBlur event... So, I need to put the code on the onFocus event of the other (end date) field... So far, it works...
2- BUT, it looks like since there's an event attached to it, I can't change months or do any interaction, because when I do so it closes the calendar. Good when I actually select a date, but very bad when I want to change month or year...

Found the answer, but I still post it here in case someone else comes accross this:
I didn't see, when we click on the Behaviors IN THE PROPERTIES panel that DMXzone was collapsed, so I was doing my things in the DW's Behaviors panel...

By doing it from the Properties panel (using DMXzone Behaviors), it all works fine!

Sorry guys...