Being specific about where your systems are
For a long time, whenever we installed systems we didn't bother to be terribly specific about where they were located, which generally comes up when you get asked about time zones. For example, a lot of things default to 'America/New York', and since that's in the Eastern time zone a lot of the time we said 'sure, whatever' and went on.
The recent American DST rule change illustrates that this is now hazardous; it is now important to actually be specific about where your machines are located, because someday there may be a real difference between 'America/New York' and 'America/Toronto'.
(Yes, Canada adopted the American DST rules. This time. I'm not sure I can count on that always being the case and besides, who knows, some day New York might decide to have their own different set of DST rules.)
All of this is irritating but probably inevitable. And I shouldn't complain too loudly, since there are places with much worse tangles of DST rules.
But while I'm mumbling about timezone issues, I must grumble at all of the people who don't offer Toronto as a timezone location choice. I know, we're not all that big and important, but at the same time many of those people seem to offer Thunder Bay instead; this strikes me as at least an eccentric choice.