Daylight savings time
The rules for Daylight Savings are changing in the U.S. this year, in accordance with the Energy Policy Act of 2005.
Clocks will be set ahead one hour on the second Sunday of March instead of the current first Sunday of April. Clocks will be set back one hour on the first Sunday in November, rather than the last Sunday of October. This will make electronic clocks that had pre-programmed dates for adjusting to daylight saving time obsolete and will require updates to computer operating systems.
Without being overly dramatic, this sounds like it could require some Y2K-style remediation. Now I'm not saying anyone should rush out and buy batteries, or restock their nuclear fallout shelters. There's not likely to be any national shutdown. I'm just saying it may require a lot of extra attention to old, old systems within the next month.
The dates when Daylight Savings starts and ends haven't changed since 1966. This is before even many of the Y2K-compliant systems were created. Certainly any Y2K remediation would have been finished before this went into law. However, there may have been many systems that either didn't need to be updated for Y2K compliance, or which may have been installed since then in ignorance of this new rule.
There were some changes to DST rules in the last year or so, as regions like Indiana and Arizona now follow DST where they did not before. I don't know if this was a result of the same law. If so, then hopefully the programmers read the whole law and adapted for this at the same time.
On the other hand, preventing an exception for Indiana is more about stripping out code than modifying it. What made Y2K such a problem was all the calculations that intrinsically treated dates as numbers, making it hard to identify where the rules were broken. For instance, it's a lot easier to find and fix a program like:
if (Today >= "04/01/2007" and Today <= "04/07/2007" and DayOfWeek(Today) == "Sunday")...but not so easy if they wrote it like this:
if (var1 >= 61 and var1 <= 68 and var2 = 1)
All that to say, if you're behind on WindowsUpdates, this would be a great time to get caught up.