[Android]如何获取本周、本月、本年的起始时间

public static long getTimeOfWeekStart(){
        Calendar ca = Calendar.getInstance();
        ca.set(Calendar.HOUR_OF_DAY, 0);
        ca.clear(Calendar.MINUTE);
        ca.clear(Calendar.SECOND);
        ca.clear(Calendar.MILLISECOND);
        ca.set(Calendar.DAY_OF_WEEK, ca.getFirstDayOfWeek());
        return ca.getTimeInMillis();
    }
    
    public static long getTimeOfMonthStart(){
        Calendar ca = Calendar.getInstance();
        ca.set(Calendar.HOUR_OF_DAY, 0);
        ca.clear(Calendar.MINUTE);
        ca.clear(Calendar.SECOND);
        ca.clear(Calendar.MILLISECOND);
        ca.set(Calendar.DAY_OF_MONTH, 1);
        return ca.getTimeInMillis();
    }
    
    public static long getTimeOfYearStart(){
        Calendar ca = Calendar.getInstance();
        ca.set(Calendar.HOUR_OF_DAY, 0);
        ca.clear(Calendar.MINUTE);
        ca.clear(Calendar.SECOND);
        ca.clear(Calendar.MILLISECOND);
        ca.set(Calendar.DAY_OF_YEAR, 1);
        return ca.getTimeInMillis();
    }