WF: No calendar for a variable date

Hello,

I added a 'visible' java.util.Date in an activity of a workflow.

The variable is writable by the user, but like a String.

I was expecting that a button 'calendar' would be displayed aside the field (as for a boolean variable, a checkbox appears automatically).

But it doesn't. Is it normal ? Is there something to set in order to have the calendar in an activity without customizations ?