# Yahoo! UI Library

### datemath 2.8.1

Yahoo! UI Library > datemath > DateMath.js (source view)
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    /**
    * The datemath module provides utility methods for basic JavaScript Date object manipulation and
    * comparison.
    *
    * @module datemath
    */

    /**
    * 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();

            // 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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