计算跨度为90天的开始时间和结束时间

public static void main(String[] args){
SimpleDateFormat df = new SimpleDateFormat("yyyy/MM/dd");
Calendar caSt = Calendar.getInstance();
caSt.set(Calendar.MONTH, 4);
caSt.set(Calendar.DAY_OF_MONTH, 1);
System.out.println(df.format(caSt.getTime()));
for(int i=0;i<90-1;i++){
calendarRoll(caSt);
}
System.out.println(df.format(caSt.getTime()));
}

private static void calendarRoll(Calendar caSt){
if (caSt.get(Calendar.MONTH) == 0 && caSt.get(Calendar.DAY_OF_MONTH) == 1) {
caSt.roll(Calendar.YEAR, -1);
}
caSt.roll(Calendar.DAY_OF_YEAR, -1);
}