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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,
	
	/**
	 * Constant field representing the date in first week of January
	 * which identifies the first week of the year.
	 * <p>
	 * In the U.S, Jan 1st is normally used based on a Sunday start of week.
	 * ISO 8601, used widely throughout Europe, uses Jan 4th, based on a Monday start of week.
	 * </p>
	 * @property WEEK_ONE_JAN_DATE
	 * @static
	 * @type Number
	 */
	WEEK_ONE_JAN_DATE : 1,

	/**
	* 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:
				this._addDays(d, amount);
				// d.setDate(date.getDate() + amount);
				break;
			case this.YEAR:
				d.setFullYear(date.getFullYear() + amount);
				break;
			case this.WEEK:
				this._addDays(d, (amount * 7));
				// d.setDate(date.getDate() + (amount * 7));
				break;
		}
		return d;
	},

	/**
	 * Private helper method to account for bug in Safari 2 (webkit < 420)
	 * when Date.setDate(n) is called with n less than -128 or greater than 127.
	 * <p>
	 * Fix approach and original findings are available here:
	 * http://brianary.blogspot.com/2006/03/safari-date-bug.html
	 * </p>
	 * @method _addDays
	 * @param {Date} d JavaScript date object
	 * @param {Number} nDays The number of days to add to the date object (can be negative)
	 * @private
	 */
	_addDays : function(d, nDays) {
		if (YAHOO.env.ua.webkit && YAHOO.env.ua.webkit < 420) {
			if (nDays < 0) {
				// Ensure we don't go below -128 (getDate() is always 1 to 31, so we won't go above 127)
				for(var min = -128; nDays < min; nDays -= min) {
					d.setDate(d.getDate() + min);
				}
			} else {
				// Ensure we don't go above 96 + 31 = 127
				for(var max = 96; nDays > max; nDays -= max) {
					d.setDate(d.getDate() + max);
				}
			}
			// nDays should be remainder between -128 and 96
		}
		d.setDate(d.getDate() + nDays);
	},

	/**
	* 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 this.getDate(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. Can currently support standard
	* U.S. week numbers, based on Jan 1st defining the 1st week of the year, and 
	* ISO8601 week numbers, based on Jan 4th defining the 1st week of the year.
	* 
	* @method getWeekNumber
	* @param {Date}	date The JavaScript date for which to find the week number
	* @param {Number} firstDayOfWeek The index of the first day of the week (0 = Sun, 1 = Mon ... 6 = Sat).
	* Defaults to 0
	* @param {Number} janDate The date in the first week of January which defines week one for the year
	* Defaults to the value of YAHOO.widget.DateMath.WEEK_ONE_JAN_DATE, which is 1 (Jan 1st). 
	* For the U.S, this is normally Jan 1st. ISO8601 uses Jan 4th to define the first week of the year.
	* 
	* @return {Number} The number of the week containing the given date.
	*/
	getWeekNumber : function(date, firstDayOfWeek, janDate) {

		// Setup Defaults
		firstDayOfWeek = firstDayOfWeek || 0;
		janDate = janDate || this.WEEK_ONE_JAN_DATE;

		var targetDate = this.clearTime(date),
			startOfWeek,
			endOfWeek;

		if (targetDate.getDay() === firstDayOfWeek) { 
			startOfWeek = targetDate;
		} else {
			startOfWeek = this.getFirstDayOfWeek(targetDate, firstDayOfWeek);
		}

		var startYear = startOfWeek.getFullYear(),
			startTime = startOfWeek.getTime();

		// DST shouldn't be a problem here, math is quicker than setDate();
		endOfWeek = new Date(startOfWeek.getTime() + 6*this.ONE_DAY_MS);

		var weekNum;
		if (startYear !== endOfWeek.getFullYear() && endOfWeek.getDate() >= janDate) {
			// If years don't match, endOfWeek is in Jan. and if the 
			// week has WEEK_ONE_JAN_DATE in it, it's week one by definition.
			weekNum = 1;
		} else {
			// Get the 1st day of the 1st week, and 
			// find how many days away we are from it.
			var weekOne = this.clearTime(this.getDate(startYear, 0, janDate)),
				weekOneDayOne = this.getFirstDayOfWeek(weekOne, firstDayOfWeek);

			// Round days to smoothen out 1 hr DST diff
			var daysDiff  = Math.round((targetDate.getTime() - weekOneDayOne.getTime())/this.ONE_DAY_MS);

			// Calc. Full Weeks
			var rem = daysDiff % 7;
			var weeksDiff = (daysDiff - rem)/7;
			weekNum = weeksDiff + 1;
		}
		return weekNum;
	},

	/**
	 * Get the first day of the week, for the give date. 
	 * @param {Date} dt The date in the week for which the first day is required.
	 * @param {Number} startOfWeek The index for the first day of the week, 0 = Sun, 1 = Mon ... 6 = Sat (defaults to 0)
	 * @return {Date} The first day of the week
	 */
	getFirstDayOfWeek : function (dt, startOfWeek) {
		startOfWeek = startOfWeek || 0;
		var dayOfWeekIndex = dt.getDay(),
			dayOfWeek = (dayOfWeekIndex - startOfWeek + 7) % 7;

		return this.subtract(dt, this.DAY, dayOfWeek);
	},

	/**
	* 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 = this.getDate(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;
	},

	/**
	 * 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.
	 * <p>
	 * <em>NOTE:</em>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.
	 * </p>
	 * @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 (YAHOO.lang.isUndefined(d)) {
			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;
	}
};

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