java 计算时间差

计算两个时间相差30天。 

  String statime = starttime.getText();
    String endtim = endtime.getText();
    
    if(!"".equals(statime)&&!"".equals(endtim)){
     long startLong = stringToDate(statime).getTime();
     long endLong= stringToDate(endtim).getTime();     
     int a = (int) ((endLong-startLong)/(24 * 60 * 60 * 1000));     
     if(a>=31){
      MessageDialog.openConfirm(getShell(), "提示", "两者之间的期限应控制在1个月内");
      return;
     }
    }