Dates in Javascript have always been a bit of a pain to work with because of the very limited API. Take this for example:

Parsing a date string and adding a week

var d = new Date(Date.parse("15/04/2009"));
d = d.setDate(d.getDate() + 7);

The above code is painful and unintuitive.

Enter DateJS

The above can be written like:

var d = Date.parse("15/04/2009").next().week();


var d = Date.parse("15/04/2009").add(1).week();

Basically it simplifies working with dates on the client site and the feature I found most useful was being about to tell what day of the week it is:

var isWednesday = Date.parse("15/04/2009").is().Wednesday();

Be sure to checkout this useful javascript library.