Mike West — Web Application Developer

Showing Perfect Time (Unobtrusively)

Via Simon Willison’s Blogmarks, I came across an interesting idea regarding the display of timestamps on web pages, and how the nasty annoyance of time-zones can be dealt with in a fairly elegant manner. In a nutshell, “Showing Perfect Time” describes a method of displaying timestamps to visitors in the visitor’s own time zone, using JavaScript to convert a seconds-since-the-GMT-epoch stamp into something pretty and localized.

I love the idea, but the implementation doesn’t work for me. Importantly, it relies on <noscript> to display times for users without JavaScript turned on and document.write to dump out times for those who do. I’m not a fan of this method, so I stood on the author’s shoulders (who, by the way, was standing on shoulders already), and wrote my own version.

PerfectTime.js is a self-contained JavaScript class that runs onload, and unobtrusively replaces the contents of a specially constructed span with a properly formatted timestamp. The code on the webpage might look something like

<span class='PerfectTime' gmt_time='1111396060'>3/21/2005 1:03 CST</span>

which I find to be more semantically meaningful, and accessible. This article details the process, but if you’d like to skip ahead and play with the code yourself, the PerfectTime JavaScript class is available for download.

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Writing this class begins in much the same way as the other classes I’ve written about on this site. We ought to map out what, exactly, we’re trying to achieve in order to give ourselves a clear goal to code towards.

The final goal is to take a GMT timestamp that’s sitting in our database, and display it to the user in her own time-zone. We know that her browser can make this translation for us if she’s using JavaScript, so we just have to figure out a nice method of making that translation possible while at the same time maintaining the semantic meaning of our page’s markup, and providing an alternative to those users visiting without JavaScript enabled. To me, this semantic HTML code looks like:

<span class='PerfectTime' gmt_time='60'>Jan 1, 1970 00:01 GMT</span>

A span surrounds the time in some baseline acceptable format (e.g. whatever you’re currently writing out), and has a custom attribute gmt_time that contains the seconds-since-epoch integer associated with that timestamp.

Without JavaScript, nothing more happens, and your users see the timestamp in some specific time-zone. With JavaScript enabled, we can take this a step further, and dynamically replace the contents of this span with a timestamp keyed off the user’s local timezone.

The steps, therefore, are straightforward:

1. Find all the spans on the page of class PerfectTime.
2. Extract the GMT stamp from each span.
3. Translate each GMT stamp to a local timestamp.
4. Replace the content of each span with the new timestamp.

So, let’s start coding.

We pick a name (“PerfectTime” sounds catchy) to distinguish this class from others we might use in the future, and create the package as per usual aliasing this to avoid scoping issues, as per usual.

function PerfectTime() {
    var self = this;

    ...

    self.instantiate = function () {
       var spans = document.getElementsByTagName('span');
       for (i=0, numSpans=spans.length; i < numSpans; i++) {
           if (spans[i].className.match(/PerfectTime/)) {
               self.processSpan(spans[i]);
           }
       }
    }

    ...

    handleEvent(window, 'load', self.instantiate);
}

We’re looking for all the spans on the page with a className of “PerfectTime”, and calling a method called (astoundingly enough) processSpan on each one. That’s where we’ll make the magic happen. As it turns out, that magic is quite straightforward:

    self.processSpan = function (theSpan) {
        var GMT = parseInt(theSpan.getAttribute('gmt_time')) * 1000;
        var newDate = new Date(GMT);
        theSpan.innerHTML = self.strftime(newDate);
    }

So, what’s going on in these three lines?

The first line grabs the gmt_time attribute off the span, and turns it into an integer using parseInt (because the getAttribute method always returns a string). We also have to multiply this number by 1000 to account for the fact that JavaScript’s Date object expects microseconds as opposed to seconds.

The second line creates our Date object, and populates it with our GMT stamp.

The third line calls a method called strftime to translate the GMT timestamp into a localized timestamp, and sticks that information into our span’s innerHTML. We’ve taken care, therefore, of steps #1, #2, and #4.
strftime() handles the heavy lifting in #3.

strftime() is based upon whytheluckystiff’s reworking of Johan Sundström’s’s clever formatTime method. I’ve simply moved the code around so that it fits into our self-contained environment. The function takes a Date object as it’s only argument, and returns the properly formatted time string just as it would have been returned from any other strftime() implementation (say, PHP’s strftime, which happens to have nicely available documentation). The formatting string is set when the class is instantiated with the following code:

self.defaultFormat = '%d %b %Y at %H:%M';
self.format        = (arguments[0])?arguments[0]:self.defaultFormat;

We use the arguments array of the constructor to determine if a formatting string was passed in. If one exists, we use it. If not, we use the default format string.

And that’s it. Simple, eh?

I’ve set up an example proof-of-concept PHP script that writes out various Unix timestamps (in fact, the timestamps of each and every blog post on mikewest.org) that will hopefully make the process clear.

The demo is PerfectTimeDemo.php. The PHP source code for the example is PerfectTimeDemo.phps. The JavaScript class is PerfectTime.js.

• Posted: 13. February 2006