Insert a test email address into the active DOM element. Update with your email user and domain. Save to your bookmarks bar.

bookmarklet.js

javascript:for(var n=new Date,m="Seconds Minutes Hours Date Month FullYear".split(" "),s="",i=m.length,a=document.activeElement;i;i--)f=n["get"+m[i-1]](),s+=10>f?"0"+f:f;if(a.isContentEditable||["input","textarea"].indexOf(a.tagName.toLowerCase())>-1)a.value="email.user+"+s+"@domain.tld";

original-script.js

(function(user, domain) {
  var now     = new Date(),
      methods = ['getFullYear', 'getMonth', 'getDate', 'getHours', 'getMinutes', 'getSeconds'].reverse(),
      string  = '',
      field;
  for (var i = methods.length; i; i--) {
    field = now[methods[i - 1]]();
    string += field < 10 ? '0' + field : field;
  }
  if (document.activeElement.isContentEditable
   || ['input', 'textarea'].indexOf(document.activeElement.tagName.toLowerCase()) > -1
  ) {
    document.activeElement.value = user + '+' + string + '@' + domain;
  }
})('email.user', 'domain.tld');