I was kind of surprised to realize, while in the bath, that simply adding 86,400 to some timestamp isn't enough to get you the next day. Because of leap seconds, you might wind up with something that's not a full day forward from the current day (in other words, the time will be different), and it's conceivable, though extremely unlikely, that you could still wind up with a timestamp representing the same day.
If your language doesn't have some "date_add" function, it seems the only way to do date math, given a timestamp, is to split the timestamp apart using strftime and then stick it back together again using mktime while adding a day. Straightforward, but cumbersome. Unfortunately, it seems that's the only way to really do correct date math, assuming, of course, that the underlying system libraries account for leap seconds.
Of course, this is one of those things that's so rare you can safely ignore it, but if you're anal about correctness like me it'll probably irk you enough to do it the "right" way.
Update: Oops, not to mention daylight savings time, which can also make a day longer or shorter than 86,400 seconds. That's a much more common occurrence, of course, and it's something you shouldn't ignore in your code.
Here's a real-life bug caused by this fact.
Update: I can't believe I'm going to have to write my own function to handle this in PHP. Incidentally, Javascript has some of the best date handling support of any language.
Update: I was also worried that this would affect how I handle time conversion between local and GMT (UTC), but luckily leap seconds happen around the world at the exact same time:
Leap seconds can occur only at the end of a month, and have only ever occurred at the end of a 30 June or 31 December. Unlike leap days, they occur simultaneously worldwide; for example, a leap second on 31 December will be observed as 7:59:60 pm U.S. Eastern Standard Time.
So, it's still ok to calculate UTC with reference to my local timezone and its offset in seconds from UTC. Whew 
Update: Between some of the above updates I did a grep on the WordPress source and found that they do the same thing (add 86,400 seconds to things). Once I get my date handling resolved, I'll probably send my function over to them. If I wind up with something that works well, I should probably submit a patch to PHP as well.
Update: Hahahahaha, the EcmaScript specification stipulates that leap seconds are ignored in its date handling! See page 117.
Update: Matt linked to this, and ramin pointed out in a comment that strtotime does increments, which I didn't realize. So, I can simply say $time = strtotime('+1 day',$time); and that handles it.
Also, one of the things I've been wondering is what numeric value you get when you have a fall daylight savings time (where your time gets set back an hour). In that case, you'll have two times that are the same time (i.e. two 1:30am's). Each has a unique reference point in UTC, but strtotime has to arbitrarily pick one when you ask for a timestamp in that limbo period.
I just came across some of the wisest comments I've ever heard/read on stardom, from Sir Ian McKellen:
TW: Do you have more young fans now that you’ve done some kid-friendly movies?
IM: Oh, yes. I’m more recognized by people of all ages. I haven’t yet seen the downside of that. It’s been nothing but good. But to become famous at 64 is perhaps easier than to become famous at 18. Now I know it’s not me who’s famous. Well, I’m only famous as the man who played Gandalf. I’m famous as the man who played Magneto. But I’m not patting myself on the back. It’s luck that I got those parts, or at least good management. I didn’t go after them. They just came my way. But I would much rather be famous as Gandalf’s representative than as Ian McKellen, because people don’t come and ask me questions about my private life, which I don’t want to answer. But I’m quite happy to talk about Gandalf, as I’m doing now.
Emphasis mine.
Wired News: It's Just the 'internet' Now, via Paul. I want to agree with them and adopt the same conventions, but the Internet isn't quite the same as television. Note, the Internet. You don't say "the television", except when you're speaking about the physical box.
The Internet is a particular network, which uses TCP/IP, does name resolution with DNS, etc. In contrast, HDTV, even though it comes over cable or satellite, and uses different standards and has a different resolution than "normal" television, is still television.
Tuesday, August 17, 2004
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new⇒Perl 6 1.0 in March?
No, Parrot is not Perl 6. Parrotis a prerequisite for Rakudo Perl1.0, bu...
Andy Lester: Dec 1, 8:40pm