Determining how long ago a page was last edited

Question

I'm making a function that takes the date a page was last edited, accessed through MediaWiki's API, compares it to the current date, and returns how long ago the page was edited, e.g. 1 year/2 days/etc.

It's essentially a massive if/else statement, and I was wondering if there was a better way to implement it.

var timestamp,
    utcArr,
    today,
    utcArr = [],
    timestamp2,
    timestamp3,
    timestamp4,
    i,
    mwArr = [],
    lastEdited;

today = new Date();
utcArr.push(today.getUTCFullYear());
utcArr.push(today.getUTCMonth() + 1); // returns value 0-11
utcArr.push(today.getUTCDate());
utcArr.push(today.getUTCHours());
utcArr.push(today.getUTCMinutes());

/**
 * Time function
 * For formatting the returned value for timestamp
 * example timestamp var: '2013-04-27T14:29:53Z'
 */
function time() {
        timestamp2 = timestamp.replace(/(\:|T|Z)/g, '-'); // use g to replace every occurrence, not just one
        timestamp3 = timestamp2.split('-');
        timestamp4 = timestamp3.filter(function (e) { // remove what's left from Z
                return e;
        });
        for (i = 0; i < timestamp4.length; i++) {
                mwArr.push(parseFloat(timestamp4[i]));
        }

        /**
         * Compares dates in array and produces the difference between them
         * @todo Find a better way to do this
         */
        if ((utcArr[0] - mwArr[0]) === 0) { // years
                if ((utcArr[1] - mwArr[1]) === 0) { // months
                        if ((utcArr[2] - mwArr[2]) === 0) { // days
                                if ((utcArr[3] - mwArr[3]) === 0 { // hours
                                        if ((utcArr[4] - mwArr[4]) === 0) { // minutes
                                                lastEdited = '1 minute';
                                        } else {
                                                lastEdited = utcArr[4] - mwArr[4] + ' minutes';
                                        }
                                } else if ((utcArr[3] - mwArr[3]) === 1 && (utcArr[4] - mwArr[4]) < 59) {
                                        lastEdited = utcArr[4] - mwArr[4] + ' minutes';
                                } else if ((utcArr[3] - mwArr[3]) === 1) {
                                        lastEdited = '1 hour';
                                } else {
                                        lastEdited = utcArr[3] - mwArr[3] + ' hours';
                                }
                        } else if ((utcArr[2] - mwArr[2]) === 1 && (utcArr[3] - mwArr[3]) < 24) {
                                lastEdited = utcArr[3] - mwArr[3] + ' hours';
                        } else if ((utcArr[2] - mwArr[2]) === 1) {
                                lastEdited = 1 day
                        } else {
                                lastEdited = utcArr[2] - mwArr[2] + ' days';
                        }
                } else if ((utcArr[1] - mwArr[1]) === 1 && (utcArr[2] - mwArr[2]) < 30) { // we'll just use 30 as it's easier than coding in each month individually
                        lastEdited = utcArr[2] - mwArr[2] + ' days';
                } else if ((utcArr[1] - mwArr[1]) === 1) {
                        lastEdited = '1 month';
                } else {
                        lastEdited = utcArr[1] - mwArr[1] + ' months';
                }
        } else if ((utcArr[0] - mwArr[0]) === 1 && (utcArr[1] - mwArr[1]) < 12 ) {
                lastEdited = utcArr[1] - mwArr[1] + ' months';
        } else if ((utcArr[0] - mwArr[0]) === 1) {
                lastEdited = '1 year'
        } else {
                lastEdited = utcArr[0] - mwArr[0] + ' years';
        }

        // empty the array or everything turns out the same
        mwArr.length = 0;
}

The time function is used about half a dozen times within an AJAX request which returns the edit date in the format "2013-04-27T14:29:53Z" which is used in the function to return lastEdited which is then appended to various ids in a table. I've yet to add complete support for 1 day/month/etc. but that'll just be adding in more if/else statements.

I did consider nested switch statements, but wasn't quite sure how to go about it.

Answer 1

I looked at the source code of moment.js for inspiration:

http://momentjs.com/

And I'm trying to be as declarative as possible, using data to define the time-units I want to use:

unitMillisecondFactors = {
    'Millisecond': 1000,
    'Second': 60,
    'Minute': 60,
    'Hour': 24,
    'Day': 356,
    'Year': 10,
    'Decade': 1
};

The Javascript Date object can compute the difference between two timestamps in milliseconds. Looking at such a difference the algorithm goes like this: we currently have units Milliseconds. The factor for getting from milliseconds to seconds is 1000. is the number we'er looking at smaller then that factor? Then we're done, we return a result by rounding down and appending the unit. If we're not done, then we devide the number by the factor and try with the next unit.

function compute_time(timestamp) {
    var n = new Date() - new Date(timestamp);
    for (var unit in unitMillisecondFactors) {
        var factor = unitMillisecondFactors[unit];
        if (n < factor) {
            n = Math.floor(n);
            if (n == 1) {
                return s + "more than " + n + " " + unit + " ago\n";
            } 
            else {
                return s + "more than " + n + " " + unit + "s ago\n";
            }
        }
        n = n / factor;
    }
}

That's it. See http://jsfiddle.net/bjelline/S23V2/ for a working example with some extra output to see the steps of computation.