Get full year from given month and date

main.js

/**
 * Splits a string given in the format `31 Jan` into an array of `[0, 31]`.
 */
const parseMonthDate = (input) => {
    const [dateString, monthString] = input.split(' ');
    const date = Number(dateString);
    switch (monthString) {
        case 'Jan': return [0, date];
        case 'Feb': return [1, date];
        case 'Mar': return [2, date];
        case 'Apr': return [3, date];
        case 'May': return [4, date];
        case 'Jun': return [5, date];
        case 'Jul': return [6, date];
        case 'Aug': return [7, date];
        case 'Sep': return [8, date];
        case 'Oct': return [9, date];
        case 'Nov': return [10, date];
        case 'Dec': return [11, date];
        default: throw new Error(`Expected month, got "${monthString}"`);
    }
};

/**
 * Returns true when the given date is earlier than or equal to a specified end-date.
 */
const isPast = (dateMM, dateDD, endMM, endDD) => {
    return (
        // If the given month is in the past, or
        (dateMM < endMM) ||
        // The given month is the same as the current month, and
        // the given day is today or earlier in the month
        (dateMM === endMM && dateDD <= endDD)
    );
};

/**
 * When given a date with no year component, will compare against a known YYYY/MM/DD
 * and determine if the given date is in the current year or if it's from last year.
 * Will return a UTC date object.
 */
const getFullDate = (dateMM, dateDD, endYYYY, endMM, endDD) => {
    const dateYYYY = isPast(dateMM, dateDD, endMM, endDD) ? endYYYY : endYYYY - 1;
    return new Date(Date.UTC(dateYYYY, dateMM, dateDD, 0, 0, 0, 0));
};

////////////////////////////////////////////////////////////////////////////////////

const end = new Date(Date.UTC(2018, 1, 20)); // Today
const endYYYY = end.getUTCFullYear();
const endMM = end.getUTCMonth();
const endDD = end.getUTCDate();

const inputs = [
    '20 Feb', // Today
    '19 Feb',
    '10 Feb',
    '29 Jan',
    '1 Jan',
    '31 Dec',
    '5 Dec',
    '31 Oct',
];

const dates = inputs.map((input) => (
    getFullDate(...parseMonthDate(input), endYYYY, endMM, endDD)
));

dates.forEach((date) => console.log(date.toUTCString()));