Convert from utm to lat,lon

gistfile1.js

// include the library
<script src="lib/proj4js-combined.js"></script>  //adjust the path for your server
                                                 //or else use the compressed version

. . .

// creating source and destination Proj4js objects
// once initialized, these may be re-used as often as needed
var dest = new Proj4js.Proj('EPSG:4236');    //source coordinates will be in Longitude/Latitude
var zone29 = new Proj4js.Proj('EPSG:23029');     //destination coordinates in UTM zone 29
var zone30 = new Proj4js.Proj('EPSG:23030');     //destination coordinates in UTM zone 30
var zone31 = new Proj4js.Proj('EPSG:23031');     //destination coordinates in UTM zone 31


. . .

// transforming point coordinates
var p = new Proj4js.Point(670848, 4120235);   //any object will do as long as it has 'x' and 'y' properties
if(zone==29) {
    Proj4js.transform(zone29, dest, p);      //do the transformation.  x and y are modified in place
} else if (zone == 30) {
    Proj4js.transform(zone30, dest, p);
} else if (zone == 31) {
    Proj4js.transform(zone31, dest, p);
}

//p.x and p.y are now EPSG:4326 lat and long in degrees