Javascript assistance with dates

My date funciton does work, but if a user types 20 for the year it deafaults to 2020. This really causes problems. Can I require them to type in a four digit year?? This would solve the problem. Below is my code.

function validateBirth(fld, errMsg)

{

var stDate = new Date(fld.value);

today=new Date();

var oneday=10006060*24;

var age = Math.ceil((stDate.getTime()-today.getTime())/(oneday));

if (stDate == “NaN”)

{

alert("Please enter a valid date for " + errMsg + “.”);

fld.focus();

return false;

}

if (age > 0)

{

alert (“Date is in the future”);

return false;

}//endif

return true;

}//endfunction validateBirthdate