The current DateTickUnit implementation uses Calendar.getInstance() to get a locale dependant calendar based on the user's Locale. It is not possible to influence this.
This is not always what you want. In my case I wanted to use a Calendar that would enforce the ISO8601 definition of week numbering, so I had to specify a calendar.
The attached patch allows the caller to set the calendar that is to be used via a new public method setCalendar().