[javascript] Javascript return number of days,hours,minutes,seconds between two dates

Here's my take:

timeSince(123456) => "1 day, 10 hours, 17 minutes, 36 seconds"

And the code:

function timeSince(date, longText) {
    let seconds = null;
    let leadingText = null;

    if (date instanceof Date) {
        seconds = Math.floor((new Date() - date) / 1000);
        if (seconds < 0) {
            leadingText = " from now";
        } else {
            leadingText = " ago";
        }
        seconds = Math.abs(seconds);
    } else {
        seconds = date;
        leadingText = "";
    }

    const intervals = [
        [31536000, "year"  ],
        [ 2592000, "month" ],
        [   86400, "day"   ],
        [    3600, "hour"  ],
        [      60, "minute"],
        [       1, "second"],
    ];

    let interval = seconds;
    let intervalStrings = [];
    for (let i = 0; i < intervals.length; i++) {
        let divResult = Math.floor(interval / intervals[i][0]);
        if (divResult > 0) {
            intervalStrings.push(divResult + " " + intervals[i][1] + ((divResult > 1) ? "s" : ""));
            interval = interval % intervals[i][0];
            if (!longText) {
                break;
            }
        }
    }
    let intStr = intervalStrings.join(", ");

    return intStr + leadingText;
}