function isValidDate(date) {
var valid = true;
date = date.replace('/-/g', '');
var month = parseInt(date.substring(0, 2),10);
var day = parseInt(date.substring(2, 4),10);
var year = parseInt(date.substring(4, 8),10);
if(isNaN(month) || isNaN(day) || isNaN(year)) return false;
if((month < 1) || (month > 12)) valid = false;
else if((day < 1) || (day > 31)) valid = false;
else if(((month == 4) || (month == 6) || (month == 9) || (month == 11)) && (day > 30)) valid = false;
else if((month == 2) && (((year % 400) == 0) || ((year % 4) == 0)) && ((year % 100) != 0) && (day > 29)) valid = false;
else if((month == 2) && ((year % 100) == 0) && (day > 29)) valid = false;
else if((month == 2) && (day > 28)) valid = false;
return valid;
}
This checks for valid days in each month and for valid leap year days.