Couple of changes.
Script
<script type="text/javascript">
$(document).ready(function () {
$("input[id$='txtSessionStTime']").blur(function () {
var userInput = $(this).val().replace(/^([0-9])([0-9]):([0-9])_$/, "0$1:$2$3");
if (!userInput.match(/([0-1][0-9]|2[0-3]):[0-5][0-9]/)) {
alert('invalid Time');
$(this).val('');
return false;
} else {
$(this).val(userInput);
return true;
};
}).mask("99:99");
});
</script>
Markup
<asp:TextBox ID="txtSessionStTime" runat="server" OnTextChanged="txtSessionStTime_TextChanged"
AutoPostBack="true"></asp:TextBox>