My timesheet uploader micro-project started off as a spontaneous scratch-your-own-itch effort with very little polish. No error handling, liberal copy-paste, vague structure - you know the story. The code obviously involved some parsing and formatting of dates and times. It wasn't very complicated so I avoided the big guns, sticking to string manipulation and simple arithmetic (figuring out that 0830-1030 means 120 minutes etc.).
Timesheet entries are fed to JIRA as JSON documents via its REST API. JSON doesn't define a standard timestamp format but examples in the documentation use ISO8601 with a timezone suffix - something like 2012-08-14T08:30:00.000+0200. I dutifully slapped on the suffix as a constant to all my outgoing timestamp data.
One of the things I wanted from the script was to detect timesheet entries that were already in JIRA to prevent duplication in the worklogs. This involved downloading the worklog for the relevant issue and comparing it with the entry that was to be uploaded. When parsing timestamps in the worklog I happily ignored the timezone suffix. I figured I would always be in the same timezone as my JIRA server.
After some time I noticed this collision detection was often failing. I had no time to analyze it so I simply took care to upload my timesheet all at once and re-upload only the failed entries if there were errors. It was a pain but still much better than stuffing JIRA manually.
When I got more free time in January I discovered the true nature of the bug. Both me and my JIRA server had effectively switched timezones when the Daylight Savings Time ended in October. The script, however, still had +0200 in its outgoing timestamps. This meant that I uploaded an entry with 08:30:00.000+0200 but then it came back as 09:30:00.000+0100. Ignoring the timezone, the script detected no overlap (as long as duration was under 61 minutes) and proceeded to re-upload the entry, again with 08:30:00.000+0200 in the timestamp.
It turns out that no, I can't ignore timezones in my calculations after all. Besides, the code is already on GitHub and right now collision detection works only for the lucky users who are two hours East from UTC together with their JIRA servers. Good thing I haven't told anyone yet :-)