Last week, users of the Twitter client for Android experienced authentication problems. It was a long and lonely Sunday night for me without my Tweeps. When the issue was fixed, word on the street was that it was due to time travel, in a sense. Sunday started the first week of 2015 if you’re using ISO week numbering.

The next morning, I got my regular weekly email from our time tracking system at work, except it showed I had recorded zero hours in the previous week. Late December tends to be a quiet time, but not that quiet. Then I looked a little closer and noticed that the email was for week 2015-52. Oops!

I thought I’d take a look at the code for the report generator, and my hunch that it was also an ISO week issue was quickly confirmed. In the code, the current date was recorded and split into year and week values. Then the week value was decremented. This seemed silly to me. I changed it to first subtract a week before splitting into the year and week values. This seemed to fix…the glitch.

So what’s the lesson in all of this? First, make sure you do the math at the right time. Secondly, make sure you understand how time works. The year of the ISO week being ahead of the calendar year only happens on limited occasion. It’s not a scenario that one would think to test (though I expect a lot more tests will include it now).