看到了有人问，回答了一下，然后记录在这里。修改JS

<script>
var jform = $('#form');
var jsubmit = $('#submit');
var jsend = $('#send');
var countdownInterval = null; // 保存定时器 ID

function startCountdown(duration) {
    var endTime = Date.now() + duration * 1000; // 计算结束时间戳
    localStorage.setItem('sendCountdownEndTime', endTime); // 保存到 localStorage

    jsend.prop('disabled', true); // 禁用按钮
    if (countdownInterval) {
        clearInterval(countdownInterval); // 清理上次倒计时
    }

    countdownInterval = setInterval(function() {
        var remainingTime = Math.max(0, Math.round((endTime - Date.now())/ 1000)); // 剩余秒数
        jsend.button('<?php echo lang('user_send_sucessfully');?> ' + remainingTime + ' ');

        if (remainingTime <= 0) {
            clearInterval(countdownInterval);
            countdownInterval = null;
             jsend.button('reset');
            jsend.prop('disabled', false); // 启用按钮
            localStorage.removeItem('sendCountdownEndTime'); // 清除
        }
    }, 1000);
}


jsend.on('click', function() {
    jform.reset();
    jsend.button('loading');
    var postdata = jform.serialize();
    $.xpost(jsend.attr('action'), postdata, function(code, message) {
        if(code == 0) {
            $.alert(message);
            $('#code').focus();
            var t = 300; // 倒计时
            jsend.button('<?php echo lang('user_send_sucessfully');?> 300 ');
            jsubmit.button('reset');
            startCountdown(t);

        } else if(code < 0) {
            $.alert(message);
            jsend.button('reset');
        } else {
            jform.find('[name="'+code+'"]').alert(message).focus();
            jsend.button('reset');
        }
    });
    return false;
});

jform.on('submit', function() {
    jform.reset();
    jsubmit.button('loading');
    var postdata = jform.serialize();
    $.xpost(jform.attr('action'), postdata, function(code, message) {
        if(code == 0) {
            $.alert(message);
            jsubmit.button(message).delay(1000).location(xn.url('user-resetpw_complete'));
        } else if(xn.is_number(code)) {
            alert(message);
            jsubmit.button('reset');
        } else {
            jform.find('[name="'+code+'"]').alert(message).focus();
            jsubmit.button('reset');
        }
    });
    return false;
});
$(document).ready(function() {

        var savedEndTime = localStorage.getItem('sendCountdownEndTime');
        if (savedEndTime) {
            var endTime = parseInt(savedEndTime);
            var remainingTime = Math.max(0, Math.round((endTime - Date.now())/ 1000));

             if (remainingTime > 0) {
                startCountdown(remainingTime)
            }
            else{
                 jsend.prop('disabled', false);
            }

        }else{
             jsend.prop('disabled', false);
        }
    });

</script>

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