date.js

"use strict";

/**
 * Create a new CustomDate object.
 * @param {Number} year     4-digit year
 * @param {Number} month    1-indexed month (1 = January, etc.)
 * @param {Number} day      Day of the month (1-31)
 */
let CustomDate = function(year, month, day) {
    // Set the defaults:
    let today = new Date();
    this.day = today.getDate();
    this.month = today.getMonth() + 1;
    this.year = today.getFullYear();

    if (year instanceof CustomDate) {
        let d = year;
        this.day = d.day;
        this.month = d.month;
        this.year = d.year;
    }
    else {
        if (typeof day !== 'undefined') {
            this.day = day;
        }
        if (typeof month !== 'undefined') {
            this.month = month;
        }
        if (typeof year !== 'undefined') {
            this.year = year;
        }
    }

    function pad(value, numDigits) {
        value = value + '';
        while (value.length < numDigits) {
            value = '0' + value;
        }
        return value;
    }

    this.setDay = function(day) {
        this.day = day;
        if (this.day > this.getDaysInMonth()) {
            this.day = this.getDaysInMonth();
        }
        if (this.day < 1) {
            this.day = 1;
        }
        return this;
    };

    /**
     * Add (or subtract) a certain number of days to this date.
     */
    this.addDays = function(num) {
        this.day += num;
        if (num < 0) {
            while (this.day <= 0) {
                this.month--;
                if (this.month < 1) {
                    this.year--;
                    this.month = 12;
                }

                this.day = this.getDaysInMonth() + this.day; 
            }
        }
        else {
            while (this.day > this.getDaysInMonth()) {
                this.day = this.day - this.getDaysInMonth();
                
                this.month++;
                if (this.month > 12) {
                    this.year++;
                    this.month = 1;
                }
            }
        }
        return this;
    };

    this.setMonth = function(month) {
        this.month = month;
        if (this.month > 12) {
            this.month = 12;
        }
        if (this.month < 1) {
            this.month = 1;
        }
        return this;
    };

    /**
     * Add (or subtract) a certain number of months to this date. 
     */
    this.addMonths = function(num) {
        this.month += num;
        if (num < 0) {
            while (this.month <= 0) {
                this.month += 12;
                this.year--;
            }
        }
        else if (num > 0) {
            while (this.month > 12) {
                this.month -= 12;
                this.year++;
            }
        }

        // If the new month has fewer days than the original, use the last day.
        if (this.day > this.getDaysInMonth()) {
            this.day = this.getDaysInMonth();
        }
        return this;
    };

    this.setYear = function(year) {
        this.year = year;
        return this;
    };

    /**
     * Add a certain number of years to this date. 
     */
    this.addYears = function(num) {
        this.year += num;
        // Handle leap years:
        if (this.day > this.getDaysInMonth()) {
            this.day = this.getDaysInMonth();
        }
        return this;
    };
    
    /**
     * Get the Javascript Date() representation of this date.
     */
    this.toDate = function() {
        let d = new Date();
        d.setFullYear(this.year);
        d.setMonth(this.month - 1, this.day);
        d.setDate(this.day);
        return d;
    };

    /**
     * Get a numeric representation of this date as a day of the week,
     * where 1 = Monday and 7 = Sunday.
     */
    this.getDayOfWeek = function() {
        // Wrap 0 (Sunday in js representation) to 7.
        let day = this.toDate().getDay();
        return day === 0 ? 7 : day;
    };

    /**
     * Get the number of days in the month specified by this date.
     */
    this.getDaysInMonth = function() {
        return new Date(this.year, this.month, 0).getDate();
    };

    /**
     * Get the day of the week as a string for display.
     */
    this.getDisplayDayOfWeek = function() {
        switch (this.getDayOfWeek()) {
            case 1: return 'Monday';
            case 2: return 'Tuesday';
            case 3: return 'Wednesday';
            case 4: return 'Thursday';
            case 5: return 'Friday';
            case 6: return 'Saturday';
            case 7: return 'Sunday';
        }
    };

    this.getDisplayDayOfWeekThreeLetter = function() {
        switch (this.getDayOfWeek()) {
            case 1: return 'Mon';
            case 2: return 'Tue';
            case 3: return 'Wed';
            case 4: return 'Thu';
            case 5: return 'Fri';
            case 6: return 'Sat';
            case 7: return 'Sun';
        }
    };

    /**
     * Get the month as a string for display.
     */
    this.getDisplayMonth = function() {
        switch (this.month) {
            case 1: return 'January';
            case 2: return 'February';
            case 3: return 'March';
            case 4: return 'April';
            case 5: return 'May';
            case 6: return 'June';
            case 7: return 'July';
            case 8: return 'August';
            case 9: return 'September';
            case 10: return 'October';
            case 11: return 'November';
            case 12: return 'December';
        }
    };

    /**
     * Get the ISO 8601 display representation of this date.
     */
    this.to8601 = function() {
        return pad(this.year, 4) + '-' + pad(this.month, 2) + '-' + pad(this.day, 2);
    };

    /**
     * Get the NZ customary display representation of this date.
     * e.g. 01/12/2001
     */
    this.toNZ = function() {
        return pad(this.day, 2) + '/' + pad(this.month, 2) + '/' + this.year;
    };

    /**
     * Get the full NZ customary display representation of this date.
     * e.g. Saturday, 1 December 2001
     */
    this.toNZFull = function() {
        return this.getDisplayDayOfWeek() + ', ' + this.day + ' ' + this.getDisplayMonth() + ' ' + this.year;
    };

    /**
     * Parse a datestring in ISO 8601 format.
     */
    this.from8601 = function(str) {
        if (str === '' || typeof str === 'undefined') return;
    
        let parts = str.split('-');
        this.year = parseInt(parts[0]);
        this.month = parseInt(parts[1]);
        this.day = parseInt(parts[2]);
        
        return this;
    };

    /**
     * Parse a datestring in NZ customary format.
     */
    this.fromNZ = function(str) {
        if (str === '' || typeof str === 'undefined') return;

        let parts = str.split('/');

        if (parts.length < 3 || parts[0] === '' || parts[1] === '' || parts[2] === '') return;

        this.day = parseInt(parts[0]);
        this.month = parseInt(parts[1]);
        this.year = parseInt(parts[2]);

        return this;
    };
}