$.extend(Date, {

	/**
	* Constant field representing Day
	* @property DAY
	* @static
	* @final
	* @type String
	*/
	DAY : "D",

	/**
	* Constant field representing Week
	* @property WEEK
	* @static
	* @final
	* @type String
	*/
	WEEK : "W",

	/**
	* Constant field representing Year
	* @property YEAR
	* @static
	* @final
	* @type String
	*/
	YEAR : "Y",

	/**
	* Constant field representing Month
	* @property MONTH
	* @static
	* @final
	* @type String
	*/
	MONTH : "M",

	/**
	* Constant field representing one day, in milliseconds
	* @property ONE_DAY_MS
	* @static
	* @final
	* @type Number
	*/
	ONE_DAY_MS : 1000*60*60*24,
  
	/**
	* Constant field mapping day strings to integers (0-6)
	* @property SUN, MON, TUE, WED, THU, FRI, SAT
	* @static
	* @final
	* @type Number
	*/  
  SUN : 0,
  MON : 1,
  TUE : 2,
  WED : 3,
  THU : 4,
  FRI : 5,
  SAT : 6,

	/**
	* Retrieves a JavaScript Date object representing January 1 of any given year.
	* @method getJan1
	* @param {Number} calendarYear		The calendar year for which to retrieve January 1
	* @return {Date}	January 1 of the calendar year specified.
	*/
	getJan1 : function(calendarYear) {
		return Date.getDate(calendarYear,0,1);
	},

	/**
	 * Returns a new JavaScript Date object, representing the given year, month and date. Time fields (hr, min, sec, ms) on the new Date object
	 * are set to 0. The method allows Date instances to be created with the a year less than 100. "new Date(year, month, date)" implementations
	 * set the year to 19xx if a year (xx) which is less than 100 is provided.
	 *


	 * NOTE:Validation on argument values is not performed. It is the caller’s responsibility to ensure
	 * arguments are valid as per the ECMAScript-262 Date object specification for the new Date(year, month[, date]) constructor.
	 * 

	 * @method getDate
	 * @param {Number} y Year.
	 * @param {Number} m Month index from 0 (Jan) to 11 (Dec).
	 * @param {Number} d (optional) Date from 1 to 31. If not provided, defaults to 1.
	 * @return {Date} The JavaScript date object with year, month, date set as provided.
	 */
	getDate : function(y, m, d) {
		var dt = null;
		if (typeof d=="undefined") {
			d = 1;
		}
		if (y >= 100) {
			dt = new Date(y, m, d);
		} else {
			dt = new Date();
			dt.setFullYear(y);
			dt.setMonth(m);
			dt.setDate(d);
			dt.setHours(0,0,0,0);
		}
		return dt;
	}
});

$.extend(Date.prototype, {

	/**
	* Adds the specified amount of time to the this instance.
	* @method add
	* @param {String} field	The field constant to be used for performing addition.
	* @param {Number} amount	The number of units (measured in the field constant) to add to the date.
	* @return {Date} The resulting Date object
	*/
	add : function(field, amount) {
		var d = new Date(this.getTime());

		switch (field) {
			case Date.MONTH:
				var newMonth = this.getMonth() + amount;
				var years = 0;

				if (newMonth < 0) {
					while (newMonth < 0) {
						newMonth += 12;
						years -= 1;
					}
				} else if (newMonth > 11) {
					while (newMonth > 11) {
						newMonth -= 12;
						years += 1;
					}
				}

				d.setMonth(newMonth);
				d.setFullYear(this.getFullYear() + years);
				break;
			case Date.DAY:
				d.setDate(this.getDate() + amount);
				break;
			case Date.YEAR:
				d.setFullYear(this.getFullYear() + amount);
				break;
			case Date.WEEK:
				d.setDate(this.getDate() + (amount * 7));
				break;
		}
		return d;
	},

	/**
	* Subtracts the specified amount of time from the this instance.
	* @method subtract
	* @param {Number} field	The this field constant to be used for performing subtraction.
	* @param {Number} amount	The number of units (measured in the field constant) to subtract from the date.
	* @return {Date} The resulting Date object
	*/
	subtract : function(field, amount) {
		return this.add(field, (amount*-1));
	},

	/**
	* Determines whether a given date is before another date on the calendar.
	* @method before
	* @param {Date} compareTo	The Date object to use for the comparison
	* @return {Boolean} true if the date occurs before the compared date; false if not.
	*/
	before : function(compareTo) {
		return (this.getTime() < compareTo.getTime());
	},

	/**
	* Determines whether a given date is after another date on the calendar.
	* @method after
	* @param {Date} compareTo	The Date object to use for the comparison
	* @return {Boolean} true if the date occurs after the compared date; false if not.
	*/
	after : function(compareTo) {
		return (this.getTime() > compareTo.getTime());
	},
  
	/**
	* Determines whether a given date is equal to another date on the calendar.
	* @method equals
	* @param {Date} compareTo	The Date object to use for the comparison
	* @return {Boolean} true if the dates are the same; false if not.
	*/
	equals : function(compareTo) {
		return (this.getTime() == compareTo.getTime());
	},

	/**
	* Determines whether a given date is between two other dates on the calendar.
	* @method between
	* @param {Date} dateBegin	The start of the range
	* @param {Date} dateEnd		The end of the range
	* @return {Boolean} true if the date occurs between the compared dates; false if not.
	*/
	between : function(dateBegin, dateEnd) {
		return (this.after(dateBegin) && this.before(dateEnd));
	},

	/**
	* Calculates the number of days the specified date is from January 1 of the specified calendar year.
	* Passing January 1 to this function would return an offset value of zero.
	* @method getDayOffset
	* @return {Number}	The number of days since January 1 of the given year
	*/
	getDayOffset : function() {
		var beginYear = Date.getJan1(this.getFullYear()); // Find the start of the year. This will be in week 1.

		// Find the number of days the passed in date is away from the calendar year start
		return Math.ceil((this.getTime() - beginYear.getTime()) / Date.ONE_DAY_MS);
	},

	/**
	* Calculates the week number for the given date. This function assumes that week 1 is the
	* week in which January 1 appears, regardless of whether the week consists of a full 7 days.
	* The calendar year can be specified to help find what a the week number would be for a given
	* date if the date overlaps years. For instance, a week may be considered week 1 of 2005, or
	* week 53 of 2004. Specifying the optional calendarYear allows one to make this distinction
	* easily.
	* @method getWeekNumber
	* @return {Number}	The week number of the given date.
	*/
	getWeekNumber : function() {
		var date = this.clearTime();
		var nearestThurs = new Date(date.getTime() + (4 * Date.ONE_DAY_MS) - ((date.getDay()) * Date.ONE_DAY_MS));

		var jan1 = Date.getJan1(nearestThurs.getFullYear());
		var dayOfYear = ((nearestThurs.getTime() - jan1.getTime()) / Date.ONE_DAY_MS) - 1;

		return Math.ceil((dayOfYear)/ 7);
	},

	/**
	* Determines if a given week overlaps two different years.
	* @method isYearOverlapWeek
	* @return {Boolean}	true if the date overlaps two different years.
	*/
	isYearOverlapWeek : function() {
		var nextWeek = this.add(Date.DAY, 6);
		return (nextWeek.getFullYear() != weekBeginDate.getFullYear());
	},

	/**
	* Determines if a given week overlaps two different months.
	* @method isMonthOverlapWeek
	* @return {Boolean}	true if the date overlaps two different months.
	*/
	isMonthOverlapWeek : function() {
		var nextWeek = this.add(Date.DAY, 6);
		return (nextWeek.getMonth() != weekBeginDate.getMonth());
	},

	/**
	* Gets the first day of a month containing a given date.
	* @method findMonthStart
	* @return {Date}		The JavaScript Date representing the first day of the month
	*/
	findMonthStart : function(date) {
		return Date.getDate(this.getFullYear(), this.getMonth(), 1);
	},
  
	/**
	* Gets the last day of a month containing a given date.
	* @method findMonthEnd
	* @return {Date}		The JavaScript Date representing the last day of the month
	*/
	findMonthEnd : function() {
		var start = this.findMonthStart();
		var nextMonth = start.add(Date.MONTH, 1);
		return nextMonth.subtract(Date.DAY, 1);
	},

	/**
	* Clears the time fields from a given date, effectively setting the time to 12 noon.
	* @method clearTime
	* @return {Date}		The JavaScript Date cleared of all time fields
	*/
	clearTime : function() {
		var date = new Date(this.getTime());
		date.setHours(12,0,0,0);
		return date;
	}
});