public static Calendar getThreadCalendar(TimeZone tz) {
	Calendar c = calendar.get();
	c.clear();
	if (tz == null) tz = ThreadLocalPageContext.getTimeZone();
	c.setTimeZone(tz);
	return c;
    }
public static Calendar getThreadCalendar ( TimeZone tz ) { Calendar c = calendar . get ( ) ; c . clear ( ) ; if ( tz == null ) tz = ThreadLocalPageContext . getTimeZone ( ) ; c . setTimeZone ( tz ) ; return c ; }
important:this function returns always the same instance for a specific thread, so make sure only
use one thread calendar instance at time.

@return calendar instance