How can I rewrite the ErrorMessage for a CustomValidator control on the client?

Question

I have a CustomValidator that is validating a telephone number for several different telephone numbering schemes. The client-side javascript looks like this:

Answer 1

function dateofbirth(sender, args) {

    var dtcDOB = document.getElementById('<%= dtcDOB.ClientID %>');

    var dob = new Date(dtcDOB.value);
    var currDate = new Date();

    if (dtcDOB.value == "") {
        args.IsValid = false;
        sender.textContent = "Provide DOB.";
        return;
    }

    args.IsValid = true;
}

Try sender.textContent = "your err msg". It worked for me.