JavaScript and ISO 8601 ordinal dates

iso8601_ordinal_dates.js

//------------------------------------------------------------------------------
// Constants
//------------------------------------------------------------------------------
const MIN_YEAR =1;
const MAX_YEAR = 9999;
const MIN_MONTH = 1;
const MAX_MONTH = 12;
const MIN_DAY = 1;
const MIN_HOUR = 0;
const MAX_HOUR = 24;
const MIN_MINUTE = 0;
const MAX_MINUTE = 59;
const MIN_SECOND = 0;
const MAX_SECOND = 60;

const MILLISECONDS_PER_DAY = 86400000;

/*
 * dateFromISOOrdinalDate parses and validates 
 * an ISO 8601 ordinal date string and returns a JavaScript Date.
 * ISO 8601 ordinal date format:  YYYY-ddd
 */
let dateFromISOOrdinalDate = function (isoOrdinalDate) {
    const ORDINALDATE_REGEX = /^(\d{4})-(\d{3})$/;

    const matches = isoOrdinalDate.match(ORDINALDATE_REGEX);
    if (matches === null) {
        throw new SyntaxError("Invalid ordinal date string");
    }

    const year = parseInt(matches[1]);

    const doy = parseInt(matches[2]);
    const maxDay = daysInYear(year);
    if (doy <= 0 || doy > maxDay) {
        throw new RangeError(`Day must be >= 1 and <= ${maxDay}.`);
    }

    return new Date(timeFromYear(year) + (doy-1)*MILLISECONDS_PER_DAY);
}


/*
 * formatDateAsISOOrdinalDate formats a date to an 
 * ISO 8601 ordinal date string.  
 * Note: month is 1-based (as opposed to JavaScript's 0-based).
 * January is month 1...December is month 12.
 */
let formatDateAsISOOrdinalDate = function (year, month, day) {
    if (year < MIN_YEAR || year > MAX_YEAR) {
        throw new RangeError(`Year must be >= ${MIN_YEAR} and <= ${MAX_YEAR}`);
    }
    if (month < MIN_MONTH || month > MAX_MONTH) {
        throw new RangeError(`Month must be >= ${MIN_MONTH} and <= ${MAX_MONTH}`);
    }
    const leapYear = isLeapYear(year);
    const dayCount = monthDayCount(month, leapYear);
    if (day < MIN_DAY || day > dayCount) {
        throw new RangeError(`Day must be >= ${MIN_DAY} and <= ${dayCount}`);
    }
    const doy = monthStartDay(month, leapYear) + day - 1;
    return year.toString().padStart(4, '0') + "-" + doy.toString().padStart(3, '0');
}


/*
 * formatJSDateAsISOOrdinalDate formats a JavaScript Date instance to an
 * ISO 8601 ordinal date string.
 * Note: to avoid all sorts of complications on your end, pass in a UTC date.
 */
let formatJSDateAsISOOrdinalDate = function (jsDate) {
    return formatDateAsISOOrdinalDate(jsDate.getUTCFullYear(), jsDate.getUTCMonth()+1,
        jsDate.getUTCDate());
}

/*
 * monthStartDay returns the ordinal day (nth day of year) 
 * on which a month begins.
 */
function monthStartDay(month, leapYear) {
    const leapOffset = leapYear ? 1 : 0;

    switch (month) {
        case 1:
            return 1;
        case 2:
            return 32;
        case 3:
            return 60 + leapOffset;
        case 4:
            return 91 + leapOffset;
        case 5:
            return 121 + leapOffset;
        case 6:
            return 152 + leapOffset;
        case 7:
            return 182 + leapOffset;
        case 8:
            return 213 + leapOffset;
        case 9:
            return 244 + leapOffset;
        case 10:
            return 274 + leapOffset;
        case 11:
            return 305 + leapOffset;
        case 12:
            return 335 + leapOffset;
        default:
            throw new RangeError("month must be >=1 and <= 12");
    }
}

/*
 *  How many days in a year?
 */
function daysInYear(y){
    return isLeapYear(y)?366:365;
}

/*
 * isLeapYear determines whether a given proleptic Gregorian year is a leap year.    
 */
function isLeapYear(year) {
    return (((year % 4 === 0) && (year % 100 !== 0)) || (year % 400 === 0));
}

/*
 * monthDayCount returns number of days in a given month.
 */
function monthDayCount(month, leapYear) {
    switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
            return 31;
        case 4:
        case 6:
        case 9:
        case 11:
            return 30;
        case 2:
            return leapYear ? 29 : 28;
        default:
            throw new RangeError(`Month must be >= ${MIN_MONTH} and <= ${MAX_MONTH}`);
    }
}

/*
 * Given a year, how many days since or before 1970.
 */
function dayFromYear(y){
    return (365 * (y - 1970) + Math.floor((y - 1969) / 4) - 
    Math.floor((y - 1901) / 100) + Math.floor((y - 1601) / 400))
}

/*
 * Convert daysFromYear into milleseconds...can feed into new Date()
 */
function timeFromYear(y){
    return dayFromYear(y)*MILLISECONDS_PER_DAY
}