How do I validate date

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Javascript

Only today's date or future date

function BirthDateValidation(dt) {
var dtToday = new Date();
var pastDate = new Date(Date.parse(dtToday.getMonth() + "/" + dtToday.getDate() + "/" + parseInt(dtToday.getFullYear() + 100)));
var lblmesg = document.getElementById("<%=lblMesg.ClientID%>");
if (dt <= pastDate) {
lblmesg.style.color = "red";
lblmesg.innerHTML = "Invalid BirthDate";
}
else {
lblmesg.style.color = "green";
lblmesg.innerHTML = "Valid BirthDate";
}
}

// is this wrong?

when I type 11/11/1111 ,it is display a valid, how do I modify this?