Parse dates in JavaScript

parse_date.js

function calcYear(yearRes, monthStr)
{
    const month = parseInt(monthStr);
    const curMonth = (new Date).getMonth();
    const curYear = (new Date).getFullYear();

    yearStr = yearRes === undefined
        ? calcYear(monthStr)
        : (yearRes.length === 4 ? yearRes : '20' + yearRes);

    return curMonth > month ? '' + (curYear + 1) : '' + curYear;
}

function parseDate(dateStr) // return YYYY-MM-DD
{
    const monthNames = ['___', 'jan', 'feb', 'mar', 'apr', 'may', 'jun', 'jul', 'aug', 'sep', 'oct', 'nov', 'dec'];

    // YYYY-MM-DD:
    if (dateStr.match(/\d\d\d\d-\d\d-\d\d/)) {
        return dateStr;
    }

    // MM/DD/YYYY or MM/DD/YY or MM/DD
    let res = dateStr.match(/^(\d\d?)\/(\d\d?)(\/(\d\d\d?\d?))?$/);
    if (res) {
        const monthStr = res[1].padStart(2, '0');
        const dayStr = res[2].padStart(2, '0');
        const yearStr = calcYear(res[4], monthStr);

        return `${yearStr}-${monthStr}-${dayStr}`;
    }

    // MM-DD-YYYY or MM-DD-YY or MM-DD
    res = dateStr.match(/^(\d\d?)-(\d\d?)(-(\d\d\d?\d?))?$/);
    if (res) {
        const monthStr = res[1].padStart(2, '0');
        const dayStr = res[2].padStart(2, '0');
        const yearStr = calcYear(res[4], monthStr);

        return `${yearStr}-${monthStr}-${dayStr}`;
    }

    // DD.MM.YYYY or DD.MM.YY or DD.MM
    res = dateStr.match(/^(\d\d?)\.(\d\d?)(\.(\d\d\d?\d?))?$/);
    if (res) {
        const monthStr = res[2].padStart(2, '0');
        const dayStr = res[1].padStart(2, '0');
        const yearStr = calcYear(res[4], monthStr);

        return `${yearStr}-${monthStr}-${dayStr}`;
    }

    // Nov 24, [20]24 or Nov 20 [20]24
    res = dateStr.match(/^(\w\w\w+)[, ]+(\d\d?)([, ]+(\d\d\d?\d?))?$/)
    if (res) {
        const monthIndex = monthNames.indexOf(res[1].trim().toLowerCase().substring(0, 3));
        if (monthIndex) {
            const monthStr = String(monthIndex).padStart(2, '0');
            const dayStr = res[2].padStart(2, '0');
            const yearStr = calcYear(res[4], monthStr);

            return `${yearStr}-${monthStr}-${dayStr}`;
        }
    }

    // 24 Nov, [20]24 or 24 Nov [20]24
    res = dateStr.match(/^(\d\d?)[, ]+(\w\w\w+)([, ]+(\d\d\d?\d?))?$/)
    if (res) {
        const monthIndex = monthNames.indexOf(res[2].trim().toLowerCase().substring(0, 3));
        if (monthIndex) {
            const monthStr = String(monthIndex).padStart(2, '0');
            const dayStr = res[1].padStart(2, '0');
            const yearStr = calcYear(res[4], monthStr);

            return `${yearStr}-${monthStr}-${dayStr}`;
        }
    }

    return '';
}

// test it:
console.log({
    '2024-12-25': parseDate('2024-12-25'),

    // MM/DD/YYYY or MM/DD/YY or MM/DD:
    '12/25/2024': parseDate('12/25/2024'),
    '12/25/24': parseDate('12/25/24'),
    '12/25': parseDate('12/25'),
    '01/07/2025': parseDate('01/07/2025'),
    '1/7/2025': parseDate('1/7/2025'),
    '1/7/25': parseDate('1/7/25'),
    '1/7': parseDate('1/7'),

    // MM-DD-YYYY or MM-DD-YY or MM-DD:
    '12-25-2024': parseDate('12-25-2024'),
    '12-25-24': parseDate('12-25-24'),
    '12-25': parseDate('12-25'),
    '01-07-2025': parseDate('01-07-2025'),
    '1-7-2025': parseDate('1-7-2025'),
    '1-7-25': parseDate('1-7-25'),
    '1-7': parseDate('1-7'),

    // DD.MM.YYYY or DD.MM.YY or DD.MM:
    '25.12.2024': parseDate('25.12.2024'),
    '25.12.24': parseDate('25.12.24'),
    '25.12': parseDate('25.12'),
    '07.01.2025': parseDate('07.01.2025'),
    '07.01.25': parseDate('07.01.25'),
    '7.1.2025': parseDate('7.1.2025'),
    '7.1.25': parseDate('7.1.25'),
    '7.1': parseDate('7.1'),

    'Dec 25, 2024': parseDate('Dec 25, 2024'),
    'dec 25 2024': parseDate('dec 25 2024'),
    'DEC 25 24': parseDate('DEC 25 24'),
    'December 25': parseDate('Dec 25'),
    'Jan 07, 2025': parseDate('Jan 07, 2025'),
    'january 07 2025': parseDate('jan 07 2025'),
    'JAN 07 25': parseDate('JAN 07 25'),
    'Jan 07': parseDate('Jan 07'),

    '25 Dec, 2024': parseDate('25 Dec, 2024'),
    '25, dec 2024': parseDate('25 dec 2024'),
    '25 DEC 24': parseDate('25 DEC 24'),
    '25 December': parseDate('25 December'),
    '07 Jan, 2025': parseDate('07 Jan, 2025'),
    '07 january 2025': parseDate('07 january 2025'),
    '07 JAN 25': parseDate('07 JAN 25'),
    '07, Jan': parseDate('07 Jan'),

    'something invalid': parseDate('invalid') || defaultDate,
});