Given the quite limited built-in functionality for working with dates in JavaScript, here are some bits and pieces I have found useful.

Comparing two dates by value:
oDateOne - oDateTwo === 0
(http://stackoverflow.com/a/7961381)

Checking if a string converts to a valid date:
given
var d = new Date("foo");
test using
Object.prototype.toString.call(d) === "[object Date]" && !isNaN(d.getTime())
(http://stackoverflow.com/q/1353684)

Formatting dates is easy if you’re using jQuery UI:
$.datepicker.formatDate('yy-mm-dd', new Date(2007, 1 - 1, 26));
(http://stackoverflow.com/a/7022296)
Alternatively, the date formatting capabilities of Moment.js are very useful.