For a client's website, I needed to enumerate the 12 months preceding a given date to create links to archived content. The site uses a javascript templating engine to create HTML, offloading the process from the server, so generating the list of months on the client side in javascript seemed like a reasonable choice. For the past week, everything looked great, but suddenly today I noticed that it was repeating the current month.

The code itself is pretty simple. It's written as a dustjs helper function that uses the Date.setMonth function to handle wrapping from January of one year to December of the previous year.

dust.helpers.month_list = function(chunk, ctx, bodies, params) {
    var count = dust.helpers.tap(params.count, chunk, ctx) | 0;
    var curDate = new Date();

    for (var i = 0; i < count; ++i) {
        var dateStr = (1900 + curDate.getYear()) + '-' + (curDate.getMonth()+1);
        chunk = chunk.render(bodies.block, ctx.push({date : dateStr}));

    return chunk;

Some quick printf debugging, adding console.log(curDate) at the start of the loop, shows this surprising result:

Tue Dec 31 2013 20:47:14 GMT-0800 (Pacific Standard Time)
Sun Dec 01 2013 20:47:14 GMT-0800 (Pacific Standard Time)
Fri Nov 01 2013 20:47:14 GMT-0700 (Pacific Daylight Time)
Tue Oct 01 2013 20:47:14 GMT-0700 (Pacific Daylight Time)

Apparently, on the 31st of the month, subtracting one month from the current date does not have the desired effect, in Chrome (on Windows 8.1). I ran the test again in IE 11 and observed the same behavior, as well as tried by manually setting the date to the 31st of October and subtracting a month, again seeing the same behavior. I'm not sure if that means this is somehow part of the specification, or if it's a bug caused by a library used underneath the hood in both browsers, but the end result is the same. My "clean" code to use setMonth(getMonth()-1) instead of writing out an if statement to detect the start of the year and wrap correctly now contains a cryptic loop that detects if the month didn't actually change and subtracts it again, all to deal with a bug that only happens once a month.

Post-migration update: The original comments for this post discussed the solution a bit and concluded that the best approach is to use curDate.setDate(0) to get the previous month, per the MDN documentation. The comments are lost to the internet now but I felt the correct solution was important enough to include in an update here.