HTML Defina o tipo de entrada Data para desativar as datas anteriores
$(function(){
var dtToday = new Date();
var month = dtToday.getMonth() + 1;
var day = dtToday.getDate();
var year = dtToday.getFullYear();
if(month < 10)
month = '0' + month.toString();
if(day < 10)
day = '0' + day.toString();
var maxDate = year + '-' + month + '-' + day;
alert(maxDate);
$('#txtDate').attr('min', maxDate);
});
RandomCoder