They both serve different purposes.

performance.now() is relative to page load and more precise in orders of magnitude. Use cases include benchmarking and other cases where a high-resolution time is required such as media (gaming, audio, video, etc.)

It should be noted that performance.now() is available in IE10 or later browsers. See caniuse page.

Date.now() is relative to the Unix epoch (1970-01-01T00:00:00Z) and dependent on system clock. Use cases include same old date manipulation ever since the beginning of JavaScript.

See When milliseconds are not enough: performance.now and now method (Internet Explorer) - MSDN for more information.

The official W3C spec can be found here: High Resolution Time API

.getMonth() returns a zero-based number so to get the correct month you need to add 1, so calling .getMonth() in may will return 4 and not 5.

So in your code we can use currentdate.getMonth()+1 to output the correct value. In addition:

• .getDate() returns the day of the month <- this is the one you want
• .getDay() is a separate method of the Date object which will return an integer representing the current day of the week (0-6) 0 == Sunday etc

so your code should look like this:

var currentdate = new Date(); 
var datetime = "Last Sync: " + currentdate.getDate() + "/"
                + (currentdate.getMonth()+1)  + "/" 
                + currentdate.getFullYear() + " @ "  
                + currentdate.getHours() + ":"  
                + currentdate.getMinutes() + ":" 
                + currentdate.getSeconds();

JavaScript Date instances inherit from Date.prototype. You can modify the constructor's prototype object to affect properties and methods inherited by JavaScript Date instances

You can make use of the Date prototype object to create a new method which will return today's date and time. These new methods or properties will be inherited by all instances of the Date object thus making it especially useful if you need to re-use this functionality.

// For todays date;
Date.prototype.today = function () { 
    return ((this.getDate() < 10)?"0":"") + this.getDate() +"/"+(((this.getMonth()+1) < 10)?"0":"") + (this.getMonth()+1) +"/"+ this.getFullYear();
}

// For the time now
Date.prototype.timeNow = function () {
     return ((this.getHours() < 10)?"0":"") + this.getHours() +":"+ ((this.getMinutes() < 10)?"0":"") + this.getMinutes() +":"+ ((this.getSeconds() < 10)?"0":"") + this.getSeconds();
}

You can then simply retrieve the date and time by doing the following:

var newDate = new Date();
var datetime = "LastSync: " + newDate.today() + " @ " + newDate.timeNow();

Or call the method inline so it would simply be -

var datetime = "LastSync: " + new Date().today() + " @ " + new Date().timeNow();

There are several ways to get the current time in JavaScript:

• new Date() creates a Date object representing current date/time
• new Date().valueOf() returns number of milliseconds since midnight 01 January, 1970 UTC
• new Date().getTime() Functionally equivalent to new Date().valueOf()
• Date.now() Functionally equivalent to the 2 methods above

As mentioned in the comments and MDN links, Date.now() is not supported in Internet Explorer 8. So if IE 8 compatibility is a concern you should use new Date().valueOf() at the cost of slightly diminished code readability.

Or if you want to use Date.now() but must be compatible with browsers which don't support it, you can place following code somewhere in your JavaScript files, which will add support for it.

if (!Date.now) {
    Date.now = function() {
        return new Date().getTime();
    }
}