Prace s globalni promennou v AJAXovem volani

var s = 0;
setInterval(function(){
  s = check(dat, u, p);
  });
}, interval);


function check(dat, u, p) {
  urlcheck = u + p;
  $.ajax({
    type: 'GET',
    url:  urlcheck,
    dataType: 'text',
    mimeType: 'text/plain',
    cache: false,
    success: function(data) {
    data = parseInt(data.replace("\r", "").replace("\n", ""));
      if(data > dat) {
        if( s==0 ) s = Date.now();
      }
    }
  });
  if(Date.now() - s > 10e3) alert('10s');
  return s;
}