Check Age Javascript function

I have a javascript function that checks to see if the person is over 18 according to the date of birth they entered in a text box.  My problem is that today's date is 3/28/2006 and if they enter a birthdate greater then 2/1/9888 it doesn't show them as 18.  In case I didn't say it right that means that if they enter 2/1/1988 it doesn't pop up the under 18 message.  If they enter 2/2/1988 it does pop up the under 18 message.  Anyone born on or before 3/28/1988 should pass the over 18 test.  Can you tell what I'm doing wrong or have a better suggestion on how to write this function?

function CheckAge(objField)
{
     var regexp = /\d{2}\/\d{2}\/\d{4}/;
     if(!objField.value.match(regexp)){
          alert('Invalid date - format : MM/DD/YYYY');
          return;
     }
     
     var day = objField.value.substr(3,2);
     var month = objField.value.substr(0,2);
     var year = objField.value.substr(6,4);
     
     var d = new Date();

     var age = d.getYear() - year;
     if(d.getMonth()/1 < month){
          age--;    
     }else if(d.getMonth()/1==month && d.getDay()/1<=day){
          age--;    
     }
      if (age < 18)
      // if under 18 display message
      {
            alert("You must be at least 18 years of age.");
            objField.focus();
            objField.select();
            return false;
      }
      return true;
}

Thanks