SimpleDateFormat is never that simple

Ah, good old days when formatting dates was easy … said no one ever.

Dealing with dates is always hard, especially when we’re talking about the limited and outdated JVM implementation in Android. Well, looking at the bright side of it, there’s always something new to be learned.

As such, I can think of no better topic than this:

You don’t really want YYYY, you want yyyy. There is a difference.

Let’s explain it a little better: SimpleDateFormat is the standard class we use to parse dates from a string of a given format into a proper EPOCH date object that we all know and love.

For its magic to work, SimpleDateFormat requires only the string template, annotated with a preset collection of letters, as per the documentation.

But there are two different ways of parsing the year, with capital Y and lower case Y, and most of the time, it doesn’t make any difference (so it’s really hard to catch in a unit test). Still, in certain conditions, it is misinterpreting the year and make a fool out of you.

As per the documentation, you should use Y to parse the string as a year counted in weeks, and y for the regular Gregorian calendar year. So what you really want is y.

The date when this bug appeared for me was on December 30th of 2019, a Monday in the standard Gregorian calendar, but that Monday also that starts the first week of 2020!

The future in the present

My parser, using YYYY parsed that date as 30-12-2020. I would not believe it if the user hadn't taken a screenshot. I was intrigued, to say the least. Gladly, there was a nice video on Youtube explaining the issue.

Now, since I know I'll forget it in the future, I'm documenting it here and hopefully saving my future self from hours of frustration. And, if you're reading this, maybe I'm saving you a few hours of frustration. You're welcome, :).

And here’s the snippet to format the date properly:

return try {
    SimpleDateFormat("dd/MM/yyyy' at 'HH:mm", Locale.getDefault()).format(Date(date))
} catch (e: ParseException) {
    null // Do proper error handling
}