# Yahoo! UI Library

### Calendar 2.3.1

Yahoo! UI Library > calendar > DateMath.js (source view)

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    /**
    * YAHOO.widget.DateMath is used for simple date manipulation. The class is a static utility
    * used for adding, subtracting, and comparing dates.
    * @namespace YAHOO.widget
    * @class DateMath
    */
    YAHOO.widget.DateMath = {
    	/**
    	* 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,

    	/**
    	* Adds the specified amount of time to the this instance.
    	* @method add
    	* @param {Date} date	The JavaScript Date object to perform addition on
    	* @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(date, field, amount) {
    		var d = new Date(date.getTime());
    		switch (field) {
    			case this.MONTH:
    				var newMonth = date.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(date.getFullYear() + years);
    				break;
    			case this.DAY:
    				d.setDate(date.getDate() + amount);
    				break;
    			case this.YEAR:
    				d.setFullYear(date.getFullYear() + amount);
    				break;
    			case this.WEEK:
    				d.setDate(date.getDate() + (amount * 7));
    				break;
    		}
    		return d;
    	},

    	/**
    	* Subtracts the specified amount of time from the this instance.
    	* @method subtract
    	* @param {Date} date	The JavaScript Date object to perform subtraction on
    	* @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(date, field, amount) {
    		return this.add(date, field, (amount*-1));
    	},

    	/**
    	* Determines whether a given date is before another date on the calendar.
    	* @method before
    	* @param {Date} date		The Date object to compare with the compare argument
    	* @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(date, compareTo) {
    		var ms = compareTo.getTime();
    		if (date.getTime() < ms) {
    			return true;
    		} else {
    			return false;
    		}
    	},

    	/**
    	* Determines whether a given date is after another date on the calendar.
    	* @method after
    	* @param {Date} date		The Date object to compare with the compare argument
    	* @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(date, compareTo) {
    		var ms = compareTo.getTime();
    		if (date.getTime() > ms) {
    			return true;
    		} else {
    			return false;
    		}
    	},

    	/**
    	* Determines whether a given date is between two other dates on the calendar.
    	* @method between
    	* @param {Date} date		The date to check for
    	* @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(date, dateBegin, dateEnd) {
    		if (this.after(date, dateBegin) && this.before(date, dateEnd)) {
    			return true;
    		} else {
    			return false;
    		}
    	},

    	/**
    	* 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 new Date(calendarYear,0,1);
    	},

    	/**
    	* 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
    	* @param {Date}	date	The JavaScript date for which to find the offset
    	* @param {Number} calendarYear	The calendar year to use for determining the offset
    	* @return {Number}	The number of days since January 1 of the given year
    	*/
    	getDayOffset : function(date, calendarYear) {
    		var beginYear = this.getJan1(calendarYear); // 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
    		var dayOffset = Math.ceil((date.getTime()-beginYear.getTime()) / this.ONE_DAY_MS);
    		return dayOffset;
    	},

    	/**
    	* 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
    	* @param {Date}	date	The JavaScript date for which to find the week number
    	* @param {Number} calendarYear	OPTIONAL - The calendar year to use for determining the week number. Default is
    	*											the calendar year of parameter "date".
    	* @return {Number}	The week number of the given date.
    	*/
    	getWeekNumber : function(date, calendarYear) {
    		date = this.clearTime(date);
    		var nearestThurs = new Date(date.getTime() + (4 * this.ONE_DAY_MS) - ((date.getDay()) * this.ONE_DAY_MS));

    		var jan1 = new Date(nearestThurs.getFullYear(),0,1);
    		var dayOfYear = ((nearestThurs.getTime() - jan1.getTime()) / this.ONE_DAY_MS) - 1;

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

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

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

    	/**
    	* Gets the first day of a month containing a given date.
    	* @method findMonthStart
    	* @param {Date}	date	The JavaScript Date used to calculate the month start
    	* @return {Date}		The JavaScript Date representing the first day of the month
    	*/
    	findMonthStart : function(date) {
    		var start = new Date(date.getFullYear(), date.getMonth(), 1);
    		return start;
    	},

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

    	/**
    	* Clears the time fields from a given date, effectively setting the time to 12 noon.
    	* @method clearTime
    	* @param {Date}	date	The JavaScript Date for which the time fields will be cleared
    	* @return {Date}		The JavaScript Date cleared of all time fields
    	*/
    	clearTime : function(date) {
    		date.setHours(12,0,0,0);
    		return date;
    	}
    };

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