gistfile1.js

 var offsets = {
     'year': 0,
     'month': 1,
     'day': 2,
     'hour': 3,
     'minutes': 4
 };

var dateTrunc = function (pred, date) {
    var trunc = offsets[pred];
    var t = Date.UTC(date.getUTCFullYear(), // 01 dev 2012 00:00:00 GMT
                     trunc >= offsets.month ? date.getUTCMonth() : 0,  // 01 Apr 2012 00:00:00 GMT 
                     trunc >= offsets.day  ? date.getUTCDate() : 1,     // 04 Apr 2012 00:00:00 GMT  
                     trunc >= offsets.hour  ? date.getUTCHours() : 0,    // 04 Apr 2012 01:00:00 GMT 
                     trunc >= offsets.minute ? date.getUTCMinutes() : 0, // 04 Apr 2012 01:59:00 GMT 
                     0,  //seconds
                     0); //microsecond

    return new(Date)(t).toISOString();
};

var date = new Date;
console.log('base', date);
console.log('year', dateTrunc('year', date));
console.log('month', dateTrunc('month', date));
console.log('day', dateTrunc('day', date));
console.log('hour', dateTrunc('hour', date));