如何用javascript判断复选框是否被选中？帮我改改以下代码

Question

if(document.Form1.CheckBox1.checked)
    {
      var Time1=QSSJ.value;
                  var Time2=JSSJ.value;
                  var Time3;
                  Time1 = Time1.replace(/-/g,"/")
                  Time2 = Time2.replace(/-/g,"/")
                  Time1 = new Date(Time1)
                  Time2 = new Date(Time2)
                  Time3 = (Time2-Time1)/(24*60*60*1000)
                  if(Time3>30)
                  {
                     alert("因过洪流量计算过程较长，选择时间请在30天以内！");
                     
                  }
 
      }

Answer 1

//if(document.Form1.CheckBox1.checked)
//modify to
if(document.getElementById("CheckBox1").checked)

Answer 2

if(document.getElementById("CheckBox1").checked)

Answer 3

document.all.CheckBox1.checked用all最保险。

Answer 4

var obj=document.getElementById("checkboxID");
if(obj.checked=true)
{
// other operation.
}