Validate Date in Javascript

Format -: mm/dd/yyyy

function checkdate(input){
var validformat=/^\d{2}\/\d{2}\/\d{4}$/
if (!validformat.test(input.value))
alert("Invalid Date Format. Please correctand submit again.")
else{
var monthfield=input.value.split("/")[0]
var dayfield=input.value.split("/")[1]
var yearfield=input.value.split("/")[2]
var dayobj = new Date(yearfield, monthfield-1, dayfield)
if ((dayobj.getMonth()+1!=monthfield)||dayobj.getDate()!=dayfield)||dayobj.getFullYear()!=yearfield))
alert("Invalid Day, Month,or Year rangedetected. Please correctand submit again.")
else
returnval=true
}
if(returnval==false)
input.select()
return returnval
}