writing a simple helper module

The plan is to write a module and include it in custom build with JSMODULESOURCES to have all nice tools in place without wasting time on load.

Starting with a time helper as sample, I was fiddling around and have no clue how to write a working module. Any suggestions how to make this work?

// module code  jstools.js

var jstools = function (){};

jstools.prototype.setTimeByUrl = function(url,tz,callback) {
	require('http').get(url, function(res){
    	var date = new Date(res.headers.Date);
    	if (date > 0 ) {
      		setTime(date/1000);
      		E.setTimeZone(tz);
      		error = null;
    	} else {
      		error = 'no date';
  		}
  		if (callback)
  			callback(error);
	});
};

exports = jstools;

// test code 
var jstools = require('jstools');

setTimeout(function(){
  jstools.setTimeByUrl('http://www.espruino.com',1,function(e){console.log(e);});
},1000);

//output 
Uncaught Error: Function "setTimeByUrl" not found!
 at line 1 col 9
jstools.setTimeByUrl('http://www.espruino.com',1,function(e)...
        ^
in function called from system