A function of date addition adddate

function | date you see the date can add, look at this code: <script language= "JavaScript" >
<!--
function Adddate (type,numday,dtdate) {
var date = new Date (dtdate)
Type = parseint (type)//type
Lintval = parseint (numday)//Interval
Switch (type) {
Case 6://year
Date.setyear (Date.getyear () + lintval)
Break
Case 7://Quarterly
Date.setmonth (Date.getmonth () + (Lintval * 3))
Break
Case 5://month
Date.setmonth (Date.getmonth () + lintval)
Break
Case 4://day
Date.setdate (date.getdate () + lintval)
Break
Case 3://
Date.sethours (date.gethours () + lintval)
Break
Case 2://Points
Date.setminutes (date.getminutes () + lintval)
Break
Case 1://sec
Date.setseconds (Date.getseconds () + lintval)
Break
Default

}
return date.getyear () + '-' + (Date.getmonth () +1) + '-' +date.getdate () + ' + date.gethours () + ': ' +date.getminutes () + ': ' +d Ate.getseconds ()
}
document.write (Adddate ("5", 5, "2005/5/21 00:00:00"))
-->
</script>