JS自动倒计时30秒后按钮才可用(两种场景)

<form action="https://www.jb51.net/" method="post" name="agree"> 
 <input type="submit" class="button" value="请认真查看<服务条款和声明> ()" name="agreeb"> 
</form> 

<script> 
var secs = ; 
document.agree.agreeb.disabled=true; 
for(var i=;i<=secs;i++) { 
 window.setTimeout("update(" + i + ")", i * ); 
} 
function update(num) { 
 if(num == secs) { 
 document.agree.agreeb.value =" 我 同 意 "; 
 document.agree.agreeb.disabled=false; 
 } 
 else { 
 var printnr = secs-num; 
 document.agree.agreeb.value = "请认真查看<服务条款和声明> (" + printnr +")"; 
 } 
} 
</script> 

<form action="https://www.jb51.net/" method="post" name="myform"> 
 <input type="button" class="button" value="获取手机验证码" name="phone" onclick="showtime()"> 
</form> 

<script> 
function showtime(t){ 
 document.myform.phone.disabled=true; 
 for(i=;i<=t;i++) { 
 window.setTimeout("update_p(" + i + ","+t+")", i * ); 
 } 
 
} 
function update_p(num,t) { 
 if(num == t) { 
 document.myform.phone.value =" 重新发送 "; 
 document.myform.phone.disabled=false; 
 } 
 else { 
 printnr = t-num; 
 document.myform.phone.value = " (" + printnr +")秒后重新发送"; 
 } 
} 
</script>