setTimeout(showT(t-1),5000)

setTimeout(function(){showT(t-1)},1000)

setTimeout("Talert(i)",5000);

<<input type="button" id="sjyzm-btn" class="sjyzm-btn" value="获取验证码"/>

 var t = 120;
 　　showT(t);
  function showT(t) {
    if(t > 0){
     $(".sjyzm-btn").val(t +"秒");
     setTimeout(function(){showT(t-1)},1000)
     //setTimeout(showT(t-1),5000)
     //$('.sjyzm-btn').attr("disabled",true);
     }else{
     $(".sjyzm-btn").val("短信验证");
     //$(".sjyzm-btn").removeAttr("disabled");
    }
   }