How many hours are there in a day? Twenty-four of course, except for the couple of days a year when there's 23 or 25. Worse, which days have odd hours changes every year, is unpredictable, and is subject to the whim of local politicians, religious leaders, and sporting events. This fall's DST change happens at 50 different times depending on your location. It's a terrible way to keep time.

Daylight Savings Time is confusing enough for people but it plays hell with software. There's an Internet advertising company I'm intimately familiar with that used to lose significant money twice a year because of customer budget algorithms that assumed every day had 24 hours. Once a year customers would only spend 23/24 of their budget losing 4% revenue, and once a year they'd spend 25/24 of their budget requiring 4% refunds. Now fixed, I'm pretty sure, but fixing it isn't easy if you're calculating in local time.

The solution for software is simple; do all calculations in Coordinated Universal Time. UTC doesn't do daylight savings, so every day has 24 hours. (Technically some days can be a second too long or short, but for most applications that error is small enough to be ignorable.) The easiest way to be sure you're doing UTC everywhere is to set your servers' timezones to UTC. You'll forever be confused when looking at log files, particularly since your own offset to UTC is changing with daylight savings, but with a desktop clock showing UTC you can usually puzzle it out.

For our recent trip Ken and I took our iPhones. It's nice to stay in touch back home and I've become pretty reliant on my iPhone for getting around. Here's some tips on using an iPhone internationally for Americans stuck with AT&T. (There's a whole alternative of unlocking the phone and using a European SIM; not discussed here). See also AT&T's iPhone travel tips, they're pretty useful.

AT&T's roaming is pretty unreliable. Half the incoming phone calls I know about never arrived, not even to voicemail. Caller ID doesn't work. SMS messages disappear. Rates are outrageous, something like $2/minute. It's a bit cheaper if you set up "AT&T World Connect" on your account for $4/month before you leave. But who cares about the phone, you really want your iPhone as an Internet terminal. And it works pretty well for that in Europe, provided you either find free WiFi or else you don't mind paying through the nose.

If you don't buy the "Data Global Add-On" you will be paying $10/mb for data from the cell network (EDGE or 3G). That's absolutely hideous pricing. You can pre-order a chunk of 20, 50, 100, or 200 mb/month at roughly $1 / megabyte. That's still outrageous but bearable. Note: you can order the data for just a few weeks and cancel. Overage is $5/mb.

How much bandwidth do you need? If I was being careful, it was 2 mb/day. That was enough bandwidth to catch up on email and Twitter twice a day, maybe get a couple of web pages or upload a photo. Then I had one bad day where I looked up a few maps and restaurants and blew through 10 megabytes. You really have to be careful.

How do you be careful? #1 way is to find free WiFi. There's a lot of free WiFi in Europe, more-so than the US. Public town squares, train stations, and busy cafes are good bets. Sometimes the cafe WiFi is password protected, just ask and they'll give you a password. Hotels are hit and miss. A lot of my rooms only had wired Internet, I regretted not bringing a little WiFi router.

If you're stuck with using cell for data, the #1 option for being careful is to turn off "Data Roaming" in Settings. If you do that, you'll use no bandwidth. You'll also not be able to use the Internet. Geolocation won't work well either. I found it really irritating to have Data Roaming off all the time. So instead I configured the phone to be lean by turning off Notifications and "Mail / Fetch New Data" in Settings. I also reset my Usage counters to track what I was using in Europe.

iPhone apps will still use a lot of bandwidth when you launch them. The real killer is Google Maps, a total hog. OffMaps is a nice alternative with cached maps in offline mode. Expect to pre-load 500-1000mb of maps for a long trip, you really want the finest grain detail when walking around. Apple's Mail app is reasonably network efficient, particularly compared to Gmail in Safari. The New York Times app is great for caching a bunch of news articles on WiFi then reading them later. And Twitter is a great low bandwidth way to keep in touch with people, although apps like Tweetie are not particularly network efficient.

AT&T's data roaming charges are ridiculous and it's a real pain watching your bandwidth. But it's totally worth it, the iPhone makes a great little companion while travelling. I particularly liked being able to use Twitter as a travel diary complete with photos, it was a lot of fun seeing responses from people to what I was doing that day.

We're back from our 25 day trip to central Europe. Ken and I were invited to a friend's wedding in Spišská Nová Ves, Slovakia. That seemed a great excuse to travel around a bunch of Central Europe. So we visited 13 cities in 4 countries. Here's some quick impressions, along with a map of where we stayed (you may need to zoom out to see everything).

The trip started in earnest for us in Slovakia, which we loved. It's a small country and not terribly wealthy. But it's also proudly part of the European Union and people were very friendly and welcoming, even flattered to be visited. Bratislava is a great international city. The smaller towns we stayed in were a bit disappointing (I'm sorry we didn't stay in Banská Bystrica instead), but the High Tatra mountains were phenomenal. And the high point of our whole trip was my friend's wedding in Spišská Nová Ves: nice family and friends, beautiful part of the country, great folkoric wedding elements. Really glad we had the opportunity to participate in such a special event.

From Slovakia we headed into southwestern Poland. Kraków was everything we hoped for and more, a beautifully preserved old town full of things to enjoy. (Although I suggest skipping the salt mine.) And Wrocław was a nice surprise, it's a university town with a tranquil old cathedral district. But while we had a good time in Poland it was probably the least friendly of all the places we went. Also the roads are terrible for driving.

After Poland we went southwest into the Czech Republic, stopping for a night in a ridiculously luxurious hotel and then spending time in Prague with an old Parisian friend. Prague is great, beautiful and overwhelming with so much to do. I wish I had a warm relaxed week to enjoy it instead of two bitterly cold days. Will definitely be back. We also spent a couple of days in Český Krumlov, a well-preserved medieval town that's definitely worth one tourist night but probably not two. Overall we liked our time in Czech, just wish I'd seen more of it. It seems very Western to us, like it's already made the post-Soviet transition that Slovakia is working so hard to make now.

We started and ended the trip in Austria, familiar territory for us. Honestly Vienna didn't work very well for me, restaurants, sightseeing, etc were all a struggle. I guess it's more of a place real people live than a welcoming tourist city. On the way home we also spent a couple of nights in Dürnstein, a tiny town in the Wachau wine region on the Danube. Totally beautiful area.

Overall it was a great trip, a nice sampling of some very different places. If I were doing this trip again I'd consider doing it entirely by train, skipping the smaller towns. Vienna, Bratislava, Kraków, Wrocław, Prague, Vienna again. Driving gives a lot of flexibility in seeing smaller places, but it's expensive and stressful and there's a lot to see in just the bigger cities.

Ken and I are in Wroclaw, Poland. The trip so far has been great, particularly my friend's traditional Slovakian wedding. Emacs is a terrible moblogging platform, but Twitter is working well. See @nelson for thoughts, photos, etc.