Disable datetime field in crm 4.0

to disable a crm datetime filed in crm, using Disabled=true is not able to full disable the field.The date text still
enable to edit. To fully disable the date filed use below script
//Disable
crmForm.all.DATETIME.Disabled=true;

var node = crmForm.all["DATETIME" +"img"].parentNode.previousSibling.firstChild;
node.disabled=true;
crmForm.all["DATETIME" + "img"].src = "/_imgs/btn_dis_cal.gif";
node.className = "ro";


//Use below code to enable it


crmForm.all.DATETIME.Disabled =false;
var node = crmForm.all["DATETIME" +"img"].parentNode.previousSibling.firstChild;
node.disabled =false;
crmForm.all["DATETIME" + "img"].src = "/_imgs/btn_dis_cal.gif";node.className = "";