This illustrates pretty well why Date is my least favorite part of JavaScript…

setISODate.js

if (typeof Date.prototype.setISODate !== 'function') {
  // Crude validation for the date input.
  var dateValidator = /^\d\d\d\d-[01]\d-[0-3]\d$/;

  /**
   * Set date from a ISO-formatted string, ie. 2011-11-28.
   */
  Date.prototype.setISODate = function (input) {
    if (dateValidator.test(input)) {
      var parts = input.split('-');
      // setYear uses two-digit years, setFullYear is what we want.
      this.setFullYear(parts[0]);
      // setMonth is zero-based (January is month zero), so we subtract
      // one before setting it.
      this.setMonth(parseInt(parts[1], 10) - 1);
      // setDate is a bit of a misnomer. It actually sets the day number
      // (1-31), not the full date.
      this.setDate(parts[2]);
      // set the hour to noon to avoid timezone issues.
      this.setHours(12);
    } else {
      throw 'Input to Date.setISODate was not well-formed. It should be in ISO format, eg. 2011-11-28';
    }

    return this;
  };
}