关于日期计算的问题，请教各位高手

public static Date convert(Date date) {
   Calendar c = new java.util.GregorianCalendar();
   c.setTime(date);
   if(c.get(Calendar.HOUR_OF_DAY)*60+c.get(Calendar.MINUTE)   < 4*60+15 );
    c.add(Calendar.DATE,-1);
   return c.getTime();
}