Timezone updates happen on almost a monthly basis, your computer has a file called the Timezone database which is maintained by a few volunteers. All timezone updates go through there and unless the application you wrote is written by someone who thinks they can do better than everyone involved in time-handling code so far, your application will stop using DST once the EU decides to stop using DST.

What was the y2k transition?

From two digit dates like 99 to four digit dates like 1999.

There was also the 2005 DST extension that screwed up everything because Windows XP didn't get a patch in time and everyone's calendars were screwed up for months.

Oh, it's that after all. I thought there were some timezone changes also put through in 2000 or so.

There was a sort of rivalry in the Pacific to jockey to the front by wrangling time-zones: https://www.csmonitor.com/1999/1201/p1s4.html

Other than that I don't think there was anything remarkable in play. Everyone was too busy patching systems for Y2K to deal with other issues.