Convert a date to a relative time string, such as "a minute ago", "in 2 hours", "yesterday", "3 months ago", etc.

function getRelativeTimeString(date, lang) {
    const minute = 60;
    const hour = minute * 60;
    const day = hour * 24;
    const week = day * 7;
    const month = day * 30;
    const year = day * 365;

    const time = date instanceof Date ? date.getTime() : date;
    const delta = Math.round((time - Date.now()) / 1000);
    const absoluteDelta = Math.abs(delta);
    const times = [
        [minute, "second", 1],
        [hour, "minute", minute],
        [day, "hour", hour],
        [week, "day", day],
        [month, "week", week],
        [year, "month", month],
        [Infinity, "year", year],
    ];
    
    let divider = year;
    let timeType = "year";
    for (const [num, timeInterval, div] of times) {
        if (absoluteDelta < num) {
            divider = div;
            timeType = timeInterval;
            break;
        }
    }
    const rtf = new Intl.RelativeTimeFormat(lang, { numeric: "auto" });

    return rtf.format(Math.floor(delta / divider), timeType);
}

Usage:

getRelativeTimeString(Date.now() + 1000) // in 1 second
getRelativeTimeString(Date.now() + 100000000) // tomorrow