Sunday, November 6, 2011

What happened at 1:17AM, November 6?

I just realized a weird side effect of daylight savings. In the fall, it creates an entire hour where times are non-unique. What I mean is, if I said I brushed my teeth at 1:17AM on November 6, would you know when that happened? It could be the first time 1:17 rolled around or the second.

I only thought of this as I sent a text during those ambiguous two hours and saw the timestamp. I use Google Voice, which shows the time sent next to each text. So I wondered if I sent one at 1:25 in the first hour and then at 1:17 in the second hour, would it show the one at 1:25 before the one at 1:17? I guess so.

Of course this mostly only matters if you think about software that timestamps things. Which is one reason why Unix time was invented, I guess. But I also start thinking about things like police reports or other important documents where you might say X happened at 1:17AM on the night of November 6. That doesn't specify exactly when it happened! There's no standard way to indicate what exact time you're talking about.

Again, this is one reason computers use Unix time, though I just discovered that apparently Unix time has this problem too. Its rule is that it always increases by 86,400 seconds per day. But some days are longer than others because of leap seconds. So again, we find that some times, like 915148800, are ambiguous. Now that really seems to pose a problem for software like server logging, etc. Why would you make the same mistake, Unix guys?

Note: Hmm, I wonder when I posted this? I guess we'll never know!

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